HK1142399A

HK1142399A - System and method for analyte measurement using a nonlinear sample response

Info

Publication number: HK1142399A
Application number: HK10108688.8A
Authority: HK
Inventors: Jr. Buck Harvey B.
Original assignee: F. Hoffmann-La Roche Ag
Priority date: 2007-05-09
Filing date: 2008-05-09
Publication date: 2010-12-03

Description

Systems and methods for analyte measurement using nonlinear sample response

The present invention relates to a measurement method and apparatus for measuring the concentration of an analyte in a fluid. More particularly, but not exclusively, the invention relates to a method and apparatus which can be used to measure the concentration of glucose in blood.

Measuring substance concentrations, especially in the presence of other confounding substances, is important in many fields, especially in medical diagnostics. For example, the measurement of glucose in body fluids such as blood is crucial for the effective treatment of diabetes.

Diabetes treatment generally involves two types of insulin therapy: basal and prandial. Basal insulin refers to sustained insulin, e.g., delayed release insulin, which is typically used prior to bedtime. Prandial insulin therapy provides additional doses of rapid-acting insulin to regulate blood glucose excursions caused by various factors, including the metabolism of sugars and carbohydrates. Proper regulation of blood glucose excursions requires accurate measurement of the glucose concentration in the blood. Otherwise, very serious complications can occur, including blindness and loss of circulation in the extremities, which can ultimately cause the diabetic to lose the use of his or her fingers, hands, feet, etc.

Many methods are known for measuring the concentration of an analyte, such as, for example, glucose, in a blood sample. These methods generally fall into one of two types: optical methods and electrochemical methods. Optical methods generally involve reflection or absorption spectroscopy to observe spectral changes in the agent. This change is caused by a chemical reaction that produces a color change that is indicative of the analyte concentration. Alternatively, electrochemical methods generally include a current or coulombic response that indicates the analyte concentration. See, for example, U.S. patent No. 4,233,029 to Columbus, U.S. patent No. 4,225,410 to Pace, U.S. patent No. 4,323,536 to Columbus, U.S. patent No. 4,008,448 to Muggli, U.S. patent No. 4,654,197 to Lilja et al, U.S. patent No. 5,108,564 to szominsky et al, U.S. patent No. 5,120,420 to Nankai et al, U.S. patent No. 5,128,015 to szominsky et al, U.S. patent No. 5,243,516 to White, U.S. patent No. 5,437,999 to Diebold et al, U.S. patent No. 5,288,636 to Pollmann et al, U.S. patent No. 5,628,890 to Carter et al, U.S. patent No. 5,682,884 to Hill et al, U.S. patent No. 5,727,548 to Hill et al, U.S. patent No. 5,997,817 to Crismore et al, U.S. patent No. 5,997 to fujwara, U.S. 38 6,004,441 to Hill et al, U.S. patent No. 3, U.S. 3 to shire 6,054,039, which is incorporated herein by reference in its entirety.

An important limitation of electrochemical methods for measuring the concentration of chemicals in blood is the effect of confounding variables on the diffusion of various active components of the analyte and reagents. Examples of limitations on the accuracy of blood glucose measurements include changes in blood composition or state (other than the condition being measured). For example, variations in hematocrit (red blood cell concentration) or the concentration of other chemicals in the blood can cause a signal to be generated from the blood sample. The change in bilirubin content of a blood sample is yet another example of a measurement of confounding variables in blood chemistry.

With respect to hematocrit in a blood sample, prior art methods rely on the separation of red blood cells from plasma in the sample, for example, by means of a glass fiber filter or with a reagent membrane containing a pore-forming agent that only allows plasma to enter the membrane. Separating red blood cells with a glass fiber filter increases the amount of blood sample required for the measurement, contrary to the expectations of the consumers of the test meter. Porous membranes are only partially effective in reducing the hematocrit impact and must be used in conjunction with increased delay times and/or AC measurements (see below) to achieve the desired accuracy.

Prior art methods have also attempted to reduce or eliminate hematocrit interference by applying DC measurements that include longer incubation times of the sample on the test reagent strip, thereby reducing the extent of the effect of the sample hematocrit on the measured glucose value. This approach also suffers from greatly increased testing time.

Therefore, there is a need for a system and method that accurately measures blood glucose even in the presence of confounding variables, including variations in hematocrit and other chemical concentrations in the blood. There is also a need for a system and method that accurately measures any medically significant component of any biological fluid. It is an object of the present invention to provide such a system and method.

In one embodiment, a method of determining a concentration of a medically significant component of a biological fluid is disclosed: the method comprises the following steps: applying a first signal having an AC component to the biological fluid, wherein the AC component has a magnitude sufficient to generate a Faraday current response from the biological fluid; measuring a current response to the first signal; determining a fundamental component of the current response; and determining an indication of the concentration of the medically significant component from the base component.

In another embodiment, a method of determining a concentration of a medically significant component of a biological fluid is disclosed: the method comprises the following steps: applying a first AC signal to the biological fluid, wherein the first AC signal has a magnitude sufficient to generate a Faraday current response from the biological fluid; measuring a current response to the first AC signal; determining a fundamental component of the current response; and determining an indication of the concentration of the medically significant component from the base component.

In yet another embodiment, a method of determining the glucose concentration of a blood sample is disclosed, comprising the steps of: applying a first signal having an AC component to said blood sample, wherein said AC component has a magnitude sufficient to produce a faraday current response from said blood sample; measuring a current response to the first signal; determining a fundamental component of the current response; and determining an indication of the glucose concentration from the basal component.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a graph of potential versus time showing a prior art excitation signal and a response to the prior art excitation signal obtained from a prior art electrochemical test strip.

Figure 2 is a graph of potential versus time showing the excitation potential of the first embodiment of the present invention, the faradaic response to the excitation potential obtained from an electrochemical test strip, and the fundamental components of the response.

FIG. 3 is a diagram of a real portion of each first Fourier admittance response component plotted against a hypothetical portion of each first Fourier admittance response component, in accordance with a method of one embodiment of the present invention.

FIG. 4 is a plot of standard error versus actual glucose concentration for several glucose concentration measurements made according to one embodiment of the present invention (hematocrit concentration is shown as a reference quantity).

FIG. 5 is a graph of actual glucose concentration versus measured glucose concentration for several samples containing 0mg/dL bilirubin, measured in accordance with one embodiment of the present invention.

FIG. 6 is a graph of actual glucose concentration versus measured glucose concentration for several samples containing 20mg/dL bilirubin, measured in accordance with one embodiment of the present invention.

FIG. 7 is a graph of actual glucose concentration versus measured glucose concentration for several samples containing 40mg/dL bilirubin, measured in accordance with one embodiment of the present invention.

Figure 8 is a table of test data for several blood samples with 25% bilirubin obtained using one embodiment of the present invention and prior art measurement techniques.

Figure 9 is a table of test data for several blood samples with 45% bilirubin obtained using one embodiment of the present invention and prior art measurement techniques.

Figure 10 is a table of test data for several blood samples with 65% bilirubin obtained using one embodiment of the present invention and prior art measurement techniques.

FIG. 11 is a plot of glucose concentration versus measurement admittance for several blood samples, with excitation potential harmonics shown as variables.

FIG. 12 is a plot of actual glucose concentration versus measured glucose concentration for several blood samples, using the fundamental frequency component of the response.

FIG. 13 is a graph used to plot actual glucose concentration versus measured glucose concentration for several blood samples of FIG. 12, using the 4 th harmonic frequency component of the response.

FIG. 14 is a graph used to plot actual glucose concentration versus measured glucose concentration for several blood samples of FIG. 12, using the 5 th harmonic frequency component of the response.

FIG. 15 is a plot of standard error versus reference glucose for several blood samples using a 128Hz excitation signal during a 0.5 second experiment.

FIG. 16 is a plot of standard error versus reference glucose for several blood samples using a 128Hz excitation signal during a 1.0 second experiment.

FIG. 17 is a plot of standard error versus reference glucose for several blood samples using a 128Hz excitation signal in a 3.0 second experiment.

FIG. 18 is a plot of standard error versus reference glucose for several blood samples using a 3 frequency excitation signal during a 0.5 second experiment.

FIG. 19 is a plot of standard error versus reference glucose for several blood samples using a 3 frequency excitation signal in a 1.0 second experiment.

FIG. 20 is a plot of standard error versus reference glucose for several blood samples using a 3 frequency excitation signal in a 3.0 second experiment.

FIG. 21 is a plot of standard error versus reference glucose for several blood samples using DC excitation signals.

FIG. 22 is a plot of standard error versus reference glucose for several blood samples using an excitation signal comprising DC and two low potential AC frequencies.

FIG. 23 is a plot of standard error versus reference glucose for several blood samples using excitation signals including DC signals, low potential AC signals, and high potential AC signals.

FIG. 24 is a plot of standard error versus reference glucose for several blood samples using a high potential AC excitation signal.

FIG. 25 is a plot of standard error versus reference glucose for several blood samples using a high potential AC excitation signal and a low potential AC excitation signal having 2 frequencies.

FIG. 26 is a plan view of an electrode pattern for a symmetrical sensor design used for one experiment using the methods described herein.

FIG. 27 is a graph of current response versus glucose concentration for multiple excitation potentials using reagent compounds including nitrosoaniline and its derivatives.

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Alterations and modifications in the illustrated device and further applications of the principles of the invention as illustrated herein are contemplated as would normally occur to one skilled in the art to which the invention relates. In particular, although the present invention is discussed with respect to blood glucose test devices and measurement methods, it is contemplated that the present invention may be used with devices that measure other analytes and other sample types. These alternative embodiments require certain modifications to the embodiments discussed herein that would be apparent to one skilled in the art.

The system and method according to the present invention allow accurate measurement of analytes in fluids. In particular, the measurement of the analyte is accurate despite the presence of interferents, which otherwise cause errors. For example, a blood glucose meter according to the present invention measures blood glucose concentrations without the errors typically caused by variations in the hematocrit levels of the samples. Accurate measurement of blood glucose is invaluable to prevent blindness, loss of circulation and other complications of inadequate blood glucose regulation in diabetic patients. An additional advantage of the system and method according to the present invention is that the measurement can be performed much faster with much smaller sample volumes and with less complex instruments, making it more convenient for diabetics to measure their blood glucose. Likewise, accurate and rapid measurement of other analytes in blood, urine, and other biological fluids provides improved diagnosis and treatment of a wide range of medical conditions.

In the context of systems for measuring glucose, it is to be understood that electrochemical blood glucose meters typically (but not always) measure the electrochemical response of a blood sample in the presence of a reagent. The reagent reacts with glucose to produce charge carriers that are not otherwise present in the blood. As a result, the electrochemical response of blood in the presence of a given signal is intended to depend primarily on blood glucose concentration. Secondly, however, the electrochemical response of blood to a given signal depends on other factors, including hematocrit and temperature. See, for example, U.S. patent nos. 5,243,516, 5,288,636, 5,352,351, 5,385,846, 5,508,171, and 6,645,368, which discuss the confusing effects of hematocrit on blood glucose measurements, and are incorporated herein by reference in their entirety. In addition, certain other chemicals can affect the transfer of charge carriers through the blood sample, including, for example, uric acid, bilirubin, and oxygen, causing errors in glucose measurements.

One embodiment according to the present invention directed to a system and method for measuring blood glucose is generally performed by electrochemically analyzing a sample with an applied AC potential of sufficient magnitude to cause significant electrochemical reactions to occur in an electrochemical cell and to produce a faraday current response derived from the AC potential, wherein the method of analyzing the response of the cell consists of a linear analysis of the response data. Even when the cell produces a non-linear current response to the AC potential, data highly useful for determining the analyte concentration of a biological fluid sample can be found in the fundamental component of the current response (i.e., the first harmonic having the same or substantially the same frequency as the fundamental frequency of the applied AC potential) by approximating the harmonic of the fundamental frequency of the applied AC potential. In one embodiment directed to determining the glucose concentration of a blood sample, the measurement and analysis methods disclosed herein produce measurements that are relatively insensitive to hematocrit and other interferences in the blood sample.

The phase angle of the current response to relatively low frequency and low potential AC signals in the presence of a reagent containing a readily reversible redox mediator such as potassium ferricyanide can be used to obtain information about the analyte content of a fluid sample as disclosed in co-pending us application 10/688,312 (us application publication 2004/0157337) and incorporated herein by reference in its entirety. For example, in the case of a sensor with this particular reagent, the applied DC potential difference, e.g., about 300mV, is suitable for generating a Faraday response in a two-amp measurement. Similarly, an applied AC potential of, for example, about 56.56mV rms is sufficient to produce a Faraday current response. It has also been noted that different reagent compounds such as nitrosoaniline and its derivatives can be used in amperometric sensors. See, for example, U.S. Pat. Nos. 5,122,244 and 5,286,362, and pending U.S. patent applications US-2005-0013731-A1, US-2005-0016844-A1, US-2005-0008537-A1, and US-2005-0019212-A1, all of which are incorporated herein by reference. In sensors with these reagents, a relatively large DC potential difference, e.g. 450 to 550mV, should be suitably applied to the sensor in order to generate a faraday response in a two amp measurement. Referring to figure 27, in accordance with one set of test data, a reagent compound comprising nitrosoaniline and a derivative thereof is applied with a DC potential in the range of about 200mV to about 500mV sufficient to generate a faraday response in a two amp measurement system. Similarly, using reagent compounds including nitrosoaniline and its derivatives, a relatively large AC potential such as 300mV rms is suitably applied to the sensor in order to generate a suitable Faraday response. By varying the magnitude of the applied AV signal and determining the nature of the AC response, it is possible for one skilled in the art of electrochemical sensors, in conjunction with the teachings contained herein, to determine the preferred potential for any particular agent contained in the sensor. Thus, different reagents may require different applied potential thresholds to produce a useful faradaic current response.

As used herein, low potential AC excitation means that the applied AC potential is insufficient to produce a faraday current response, while high potential AC excitation means that the applied AC potential is sufficient to produce a faraday current response, each depending on the particular reagent being applied. It should be noted that in some cases, the faraday reaction in the response to a given high potential AC excitation can cause the response to have a non-linear characteristic, i.e., the applied sinusoidal waveform can produce a non-sinusoidal response.

Referring to FIG. 1, the test is performed using an electrochemical test strip constructed in accordance with the disclosure of the above-mentioned co-pending published patent application US-2005-0013731-A1. That is, test strips for conducting the tests disclosed throughout this application include ACCU manufactured and distributed by Roche diagnostics corporation, Indianapolis, IndianaAVIVA^TMAnd (3) testing the strip.

The measurements were performed using an electrochemical test stand (stand) constructed on the basis of VXI components from Agilent and programmed to apply AC and DC potentials to the sensors in the required combination and sequence and the measured sensor current response. Data transmission from electrochemical analyzer to desk top computer, applicationAnd (6) carrying out analysis. The measurements may be carried out by any commercially available programmable potentiostat and appropriate frequency response analyzer and digital signal acquisition system. For commercial applications, the method may be inSupplied low cost hand-held measuring devices such as ACCU-AVIVA^TMIn a blood glucose meter. In this case, the measurement parameters may be included in or provided to the firmware of the instrument, and the measurement sequence and data evaluation performed automatically without user interaction. For example, applying a programmable potentiostat as described above, measurements are made and the results are analyzed in such a way that: the results can be processed, made available to the user, and/or displayed to the user within about 4 seconds, about 2 seconds, or as little as about 1 second after the sample containing the analyte is applied to the biosensor and detected by the device. Similarly, ACCU-AVIVA^TMThe firmware of the blood glucose meter may be provided with measurement parameters configured and arranged such that the measurement sequence, data evaluation, and result display occur contemporaneously, i.e., about 4 seconds, about 2 seconds, or as little as about 1 second after the sample is plunged and its contact with the reagent compound is detected by the meter.

In FIG. 1, a first plot of potential versus time is shown illustrating an AC excitation potential 100 applied to an electrochemical test strip to which a whole blood sample is applied. This is typical of prior art low potential AC excitation, which is selected so as not to excite a current response on the test strip electrodes, in other words, not to be sufficient to produce a faraday current response. The excitation potential 100 is a 128Hz sinusoid with a voltage of 9mV rms. The measured response of the test strip to this challenge is also illustrated as 102. As shown, the response 102 is linear and maintains the frequency content and sinusoidal shape of the excitation potential 100, as well as having a desired phase shift.

FIG. 2 is a second graph of potential versus time illustrating a first embodiment excitation potential 200 of the present invention applied to the same type of electrochemical test strip and blood sample components used to generate the data illustrated in FIG. 1. The excitation potential 200 is also a 128Hz sinusoid, but the excitation voltage is 300mV rms, which is a high potential AC excitation sufficient to apply the particular test strip configuration and reagent composition to produce an electrochemical process and faraday current response on the test strip. Evidence of this electrochemical process is provided by the current response 202 measured on the test strip. It should be noted that the response 202 does not maintain a pure sinusoidal shape of the excitation potential, but exhibits a non-linear shape caused by the presence of higher order harmonics mixed with fundamental components having the same or substantially the same frequency as the AC excitation frequency.

Various embodiments disclosed herein apply analysis of the fundamental component of the nonlinear current response in order to accurately determine the analyte concentration of a sample, substantially independent of interferents in the sample. In one embodiment, the response 202 is measured as an admittance value and the components of the response 202 are obtained, such as by performing a fourier transform on the response 202 data, which will result in the first fourier component 204 illustrated in fig. 2. Those skilled in the art will appreciate that the first fourier component represents the fundamental component of the current response 202 (i.e., the component of the response 202 having the same or substantially the same frequency as the AC excitation frequency) and may be obtained in any of a number of ways known in the art, such as by way of a Fast Fourier Transform (FFT) or a Discrete Fourier Transform (DFT).

Once the fundamental component of the sensor current response has been determined, it is possible to calculate the impedance of the sensor or its inverse, the admittance, from the vector form of the fundamental component, the excitation potential and ohm's law (E ═ IZ). In this case, the quantities E (potential), I (current) and Z (impedance) are vectors having a magnitude and a direction. The impedance vector is typically analyzed by reference to its magnitude and phase angle. According to the vectorial form of ohm's law, the phase of the impedance is the angle between the potential vector (E) and the current vector (I).

Admittance is also a vector having a magnitude and a direction. It is sometimes convenient to analyze vectors as ordered pairs (ordered pairs) in cartesian coordinates, rather than in terms of magnitude and direction. For this purpose, the X-axis of a common cartesian coordinate plane represents the true axis along which the plotted values are referred to as the true component of the impedance or admittance, or sometimes the in-phase component. Similarly, values plotted along the Y-axis are referred to as imaginary components or anti-phase components.

Electrochemical impedance is sometimes analyzed according to an equivalent circuit model. It is a theoretical set of such electrical components: the element will have the same impedance as the electrochemical system under study when constructed and subjected to the same excitation signal. Since the electrochemical system being analyzed is not an ideal electrical element, some elements of the equivalent circuit model are not real electrical elements such as resistors and capacitors, but are mathematical descriptions such as Warburg elements for diffusion and constant phase elements that account for non-idealities of the electrode surface. An equivalent circuit model for a typical biosensor is discussed in U.S. Pat. No. 6,645,368, which is incorporated herein by reference in its entirety. ACCU-AVIVA^TMAn equivalent circuit model of the sensor is made to help evaluate the impedance data from the measurement method.

FIG. 3 illustrates admittance (Y) for a fundamental component_Hypothetical) Is drawn as the admittance (Y) of the fundamental component_{Reality (reality)}) From the analysis of 7 blood samples, each with a different glucose level. The real and imaginary parts are calculated from the measured admittance magnitude and phase angle and applying the correlations indicated in equations 1 and 2.

Y_{Reality (reality)}＝Y_{Magnitude of}*cos(Y_{Phase (C)}) (equation 1)

Y_Hypothetical＝Y_{Magnitude of}*sin(Y_{Phase (C)}) (equation 2)

Wherein:

Y_{magnitude of}Is a measure of admittance;

Y_{phase (C)}Is the phase angle of the measured admittance;

Y_{reality (reality)}Is the real part of the measurement admittance; and

Y_hypotheticalIs a hypothetical part of the measurement admittance.

As illustrated by the fit lines 300a to 300e depicted for 5 of the 7 data populations, there is a very high correlation between the real and imaginary parts of the fundamental component when the sample is excited by high potential AC excitation. Also, all the lines 300a to 300e converge at the same point. The slope of each of the lines 300 a-300 e is related to the glucose value of the test sample from which the data was generated. If the convergence intercept is ideally located at the origin (0, 0), then the parameter corresponding to the glucose value of the test sample will be tangent to the phase angle of the fundamental component of the current response to the AC excitation signal. However, since the intercept in the system that achieves these results is not the origin, the glucose value is more accurately calculated as a function of the phase angle for the current response fundamental component of the AC excitation signal, shifted to a different origin, as shown in equation 3:

glucose ═ F ((Y)_i-Y_i0)/(Y_r-Y_r0) (equation 3)

Wherein:

Y_iis the imaginary part of the admittance response of the fundamental component of the current response;

Y_i0is a hypothetical part (Y) of the offset intercept_i0，Y_r0)；

Y_rIs the true part of the admittance response of the fundamental component of the current response; and

Y_r0is the true part of the offset intercept (Y)_i0，Y_r0)。

Changing the origin of the coordinate system, i.e. determining the offset intercept of a particular analysis system, corresponds to removing components from the equivalent circuit model that are not of interest to the analyte in question. For example, the application of ACCU-AVIVA^TMThe model, the impedance of the solution resistant element and the impedance of the electrode capacitive element are removed from the equivalent circuit model of the sensor, leaving only the impedance caused by the faradaic and diffusion processes of the sensor. These values can be determined empirically by analyzing data collected from the sensor using different samples. This value can then be used to analyze data from other sensors having the same structure, reagent, and sample type. Offset intercept is generally dependent on sensor geometry and reagent factors; but the intercept can be assumed to be fixed for each specific sensor and reagent configuration. Alternatively, the offset may be determined by detecting data collected at other potentials or other frequencies, such as high frequency low potential AC measurements taken before or after or simultaneously with the low frequency high potential measurements.

A suitable new origin of the coordinate system may also be determined experimentally, as illustrated in this example. That is, the data points of the sensor trial may be plotted on a coordinate axis and a line drawn to determine the most common intersection point. This spot can then be used to analyze data from other sensors with the same structure, reagents and sample type.

FIG. 4 illustrates glucose data obtained from a covariance assay of blood samples having 5 different hematocrit levels (about 20, 35, 50, 60, and 70%) and 5 different glucose levels (about 35, 120, 330, 440, and 600mg/dL) using the method of equation (3). Using the methods disclosed herein, a blood sample is applied to a test strip containing a reagent chemical and subjected to an excitation potential large enough to induce a faraday current response. From the fundamental components of the current response data, the real and imaginary components of the admittance are plotted as described with reference to fig. 3, and the predicted glucose value for the sample is calculated as described above for equation (3). In fig. 4, the normalized glucose error is plotted against the true glucose concentration of the test sample, and the sample hematocrit concentration is shown as a reference quantity. As can be seen, the method of the present invention produces a very small extension of the standard error of the reported glucose level as the hematocrit concentration changes, indicating that the method is relatively insensitive to the hematocrit concentration in the sample. The 200 data points plotted in FIG. 4 were all within +/-15mg/dL of true glucose concentration, with the exception of 2 data points.

The systems and methods embodying the invention disclosed herein are also relatively insensitive to other interferents that typically reduce the accuracy of glucose testing on whole blood samples. For example, the method described above was used to measure glucose concentrations in a covariance study of whole blood samples having 3 different glucose concentrations (40, 120 and 450mg/dL) and 3 different bilirubin concentrations (0, 20 and 40 mg/dL). FIG. 5 illustrates the results of a study of samples with 0mg/dL bilirubin, showing the actual glucose concentration plotted against the glucose concentration measured and calculated using the method disclosed above. As seen, R²The correlation coefficient was.9901. FIG. 6 illustrates the results of a study of samples with 20mg/dL bilirubin, showing the actual glucose concentration plotted against the glucose concentration measured and calculated using the method disclosed above. As seen, R²The correlation coefficient was 996. Finally, FIG. 7 illustrates the results of a study of samples with 40mg/dL bilirubin, showing the actual glucose concentration plotted against the glucose concentration measured and calculated using the methods disclosed above. As seen, R²The correlation coefficient is.9962. It will be apparent to those skilled in the art that bilirubin levels are substantially excluded as interferents when applying the systems and methods of the present invention. Thus, the systems and methods of the present invention are useful for blood samples with potentially high bilirubin levels, such as neonatal samples.

In the application of whole blood samples and ACCU-AVIVA^TMIn another study of sensors, embodiments of the present system and method were compared to standard (prior art) DC current measurements of samples having relatively low glucose levels to determine glucose measurements. Samples with 3 different target glucose levels (ranging from 63mg/dL to 128mg/dL) and 3 different target hematocrit levels (25%, 45%, and 65%) were usedAnd (6) carrying out covariance research. For each sample, glucose concentration was measured using the system and method described herein and a standard prior art Cottrellian DC amperometric technique. The results of the tests are tabulated in FIGS. 8-10.

In FIG. 8,3 samples having glucose levels of 63mg/dL, 90mg/dL, and 126mg/dL and a target hematocrit of 25% were tested using the systems and methods described herein and the standard prior art CottrellianDC amperometry technique. Tests using systems and methods embodying the present invention were conducted at 128Hz and with a sinusoidal excitation potential of 300mV rms, and yielded calculated glucose levels with a maximum error of 5.2mg/dL from the actual change in value, with a standard deviation range of 1.303 to 2.096. In contrast, the prior art DC test produces calculated glucose levels that vary by a maximum error of 72.38mg/dL from actual values, with a standard deviation range of 9.803 to 10.472.

In FIG. 9, 3 samples having glucose levels of 67mg/dL, 89mg/dL, and 113mg/dL and a target hematocrit of 45% were tested using the systems and methods described herein and the standard prior art CottrellianDC amperometry technique. Tests using the system and method of the present invention were performed at 128Hz and with a sinusoidal excitation potential of 300mV rms, and yielded calculated glucose levels with a maximum error of 5.04mg/dL from the actual value change, with a standard deviation range of 1.159 to 2.347. In contrast, the prior art DC test produces calculated glucose levels that vary by 56.44mg/dL with a maximum error from the actual value, and a standard deviation range of 10.056 to 11.289.

In FIG. 10, 3 samples having glucose levels of 72mg/dL, 98mg/dL, and 128mg/dL and a target hematocrit of 65% were tested using the systems and methods described herein and the standard prior art CottrellianDC amperometry technique. Tests using the system and method of the present invention were conducted at 128Hz and with a sinusoidal excitation potential of 300mV rms and yielded calculated glucose levels with a maximum error of 7.93mg/dL from the actual change in level and a standard deviation range of 2.452 to 4.506. In contrast, the prior art DC test produced calculated glucose levels that varied from actual values with a maximum error of 76.44mg/dL and a standard deviation range of 10.117 to 15.647. Clearly, the present system and method provide significant improvements in accuracy (maximum error) and consistency (standard deviation) over the prior art.

Experiments were also conducted to compare systems and methods embodying the present invention for glucose calculations derived from higher order harmonics of the current response. Reuse of ACCU-AVIVA^TMThe sensor, samples with glucose levels of 11mg/dL, 122mg/dL, 333mg/dL, and 543mg/dL received a sinusoidal excitation potential of 300mV rms at a frequency of 128Hz, which was high enough to generate a Faraday current response from the test sample. Fig. 11 plots the sample admittance measured at the fundamental frequency and the second through fifth harmonic frequencies for each of the 4 glucose levels. As can be seen in the figure, only the fundamental, 4 th and 5 th harmonics show the dependency between the glucose level and the measured admittance, and therefore a more detailed study of each of these data sets was performed, as shown in fig. 12-14.

From these experiments, it is clear that while the use of fundamental components in the above-described systems and methods provides very high accuracy, in systems where the faraday current response is non-linear, other harmonic components may also be used in such systems and methods in order to provide relatively accurate analyte concentration calculations. Therefore, the application of ACCU-AVIVA^TMIn glucose measurement systems of sensor chemistry and configuration, the 4 th and 5 th harmonics can also be applied. Other harmonics may be similarly useful in other analyte systems or in other glucose systems that employ different sensor configurations. As will also be seen in the discussion of FIGS. 13 and 14 below, it is also clear that reduced accuracy, especially above certain analyte concentrations, can attenuate the application of harmonicsRather than the utility of the fundamental component. The use of harmonics does provide useful results in limited circumstances and should also be considered as an embodiment of the present invention.

FIG. 12 plots actual glucose values versus predicted glucose values (calculated using fundamental frequency data using a system and method embodying the present invention). As can be seen, the present technique provides a very accurate prediction of glucose level, the correlation coefficient (R)²) Is 0.9825. This is true both at low and high actual glucose values.

FIG. 13 plots actual glucose values versus predicted glucose values (calculated using harmonic 4 frequency data using a system and method embodying the present invention). As can be seen, the application of harmonic 4 and the inventive technique severely reduces the accuracy of the predicted glucose level, the correlation coefficient (R)²) Down to 0.8696. However, despite the overall decreased accuracy, the accuracy still appeared high between the lower actual glucose values to 333mg/dL of sample.

FIG. 14 plots actual glucose values versus predicted glucose values (calculated using harmonic 5 frequency data using a system and method embodying the present invention). As can be seen, the application of harmonic No. 5 and the inventive technique severely reduces the accuracy of the predicted glucose level, even lower than that obtained with harmonic No. 4, the correlation coefficient (R)²) Down to 0.7659. But similar to the data for harmonic 4, the accuracy still appears high at lower actual glucose values, despite the overall accuracy degradation.

As will be appreciated from the foregoing, the systems and methods of the present invention provide highly accurate analyte measurements in a biological fluid sample. The system and method of the present invention are particularly useful for the measurement of glucose concentration in a blood sample. The most accurate systems and methods embodying the invention apply to the fundamental frequency component of the current response generated from the test sample when an excitation potential large enough to generate a faraday response is applied to the sample. Although the measurements detailed above are performed at 300mV rms and 128Hz, it will be appreciated that the most useful excitation signal magnitude and frequency for any given measurement will be determined by a number of factors, including the body test strip (biosensor) design and the choice of reagents used on the test strip. The selection of the most useful potentials and frequencies for particular sensors and reagents is an optimization that can be readily accomplished by one skilled in the art without undue experimentation, in view of the guidance set forth throughout this disclosure.

Likewise, one skilled in the art will appreciate that the alternating applied potential may have many forms other than the pure sinusoidal signal used for the experiments described above. As used herein, the phrase "signal having an AC component" refers to a signal having certain alternating potential (voltage) portions. For example, the signal may be an "AC signal" with 100% alternating potential (voltage) and no DC part; the signal may have time spaced AC and DC portions; or the signal may be AC with a DC offset (AC, DC signal superposition). Also, the AC portion may include multiple frequencies that are applied sequentially at time intervals or immediately, or even simultaneously as multiple frequency signals.

With respect to the latter, the systems and methods described herein are also useful in measuring analyte concentrations in fluid samples using multiple AC excitations. For example, additional tests were conducted to demonstrate the effectiveness of the methods disclosed herein in combination with the methods disclosed in co-pending U.S. patent application publications US-2004-0157339-A1, US-2004-0157337-A1, 2004/0157338-A1, US-2004-0260511-A1, US-2004-0256248-A1, and US-2004-0259180-A1 to achieve accurate measurements in a very short period of time. This additional experiment also demonstrates the effectiveness of the method disclosed herein to obtain accurate results using a combined multi-frequency AC excitation waveform without imposing a DC offset on the AC excitation, which not only makes the measurement time short, but also makes the measurement sequence suitable because the AC signal set does not permanently alter the sensed chemistry in the manner that DC measurements do due to the alternating polarity of the applied excitation. Furthermore, according to the methods disclosed in co-pending U.S. patent application publications US-2004-0157339-A1, US-2004-0157337-A1, 2004/0157338-A1, US-2004-0260511-A1, US-2004-0256248-A1, and US-2004-0259180-A1, additional frequencies of AC signals are applied at low excitation AC potentials to produce non-faradaic current responses from which phase angles provide indications of certain interference factors from which determinations of one or more interferent corrections can be made and used to more accurately determine the analyte concentration in the fluid sample.

In this experiment, whole blood samples with 6 different target concentrations of glucose (30, 60, 90, 250, 400 and 600mg/dL) and 3 different target concentrations of hematocrit (25%, 45% and 65%) were analyzed in a covariance study. The simultaneous multifrequency excitation waveforms are generated by summing sine waves of 1 period at 300mV rms, 10 periods at 9mV rms, and 100 periods at 9mV rms. This gives three frequencies for analysis, with a frequency ratio of 1/10/100. The excitation signal is applied to the sensor at a repetition rate of 128Hz, so for analysis at the fundamental frequency, the applied and available frequencies are 128Hz, 1280Hz and 12800 Hz. Data was collected at 100ms intervals. Analysis was terminated at 100ms intervals of 500ms, 1000ms and 3000ms data.

First, a DFT (discrete fourier transform) is applied to obtain 128Hz fundamental frequency data from the measurement data. The data was analyzed using the same method as described above. That is, the real and imaginary components of the admittance are calculated and plotted against each other, the offset intercept from the natural origin is determined so that the data converges to a point, and the slope of the line connecting the offset point and the data set is determined. To accommodate the software used in the experiment, the values were converted by the following equation:

K-90-Arctan (slope) (equation 4)

Yielding a positive parameter that increases with increasing glucose. After a calibration curve for glucose was generated from the following model by non-linear fitting of the parameters intercept, slope and power,

glucose intercept + slope ^ power (equation 5)

A predicted glucose value is calculated for each measured sample and a Total Systematic Error (TSE) is calculated for each time point, as will be appreciated by those skilled in the art.

The DFT is also applied, taking data from other frequencies with low potential AC excitation from the raw signal, calculating the magnitude and phase of the admittance in the same manner as discussed above, and using it in the calibration mode.

In fig. 15-17, standard error is plotted against reference glucose values using only 128Hz data at 0.5 seconds, 1.0 seconds, and 3.0 seconds from dosimetry, respectively. Standard error is plotted against reference glucose values in fig. 18-20, using combined 128Hz, 1280Hz, and 12800Hz data at 0.5 seconds, 1.0 seconds, and 3.0 seconds, respectively. The total systematic error for each of the 6 data sets is summarized in table 1 below.

TABLE 1

Total error of system

Time of day	Only 128Hz	128Hz+1280Hz+12800Hz
Time of day	Only 128Hz	128Hz+1280Hz+12800Hz	0.5 second	17％	13％
1.0 second	14.7％	7.7％	0.5 second	17％	13％
1.0 second	14.7％	7.7％	3.0 second	34.4％	7.6％

The above experiments clearly show the feasibility of using a continuous frequency waveform as an excitation signal for simultaneously measuring an analyte and correcting for interferents in a very short time. The total measured time of the measurements compiled in table 1 may be about 4 seconds, about 2 seconds, or as low as about 1 second, including the time to process the data and display the results by a hand-held instrument or other suitable programmable potentiostat.

In conjunction with the teachings contained herein and the teachings of the methods disclosed in co-pending U.S. patent application publications US-2004-0157339-A1, US-2004-0157337-A1, 2004/0157338-A1, US-2004-0260511-A1, US-2004-0256248-A1, and US-2004-0259180-A1, electrochemistry will be able by one of ordinary skill in the art of electrochemical sensors to enhance the accuracy of analyte measurements using multi-frequency methods, using continuous frequency application. For example, applying signals from a low potential AC excitation at a higher frequency (e.g., 9mV rms, 1280Hz, and 12800Hz), followed by a high potential AC excitation at a lower frequency (e.g., 300mV rms, 128Hz, as discussed above) can enable determination of an interferent correction, which then adjusts the determination of the analyte concentration based on the interferent correction.

Yet another experiment was conducted to study systems and methods embodying the present invention when samples were excited by multiple AC frequencies and also by AC and DC excitation. Covariance studies were performed on whole blood samples with 4 different target concentrations of glucose (50, 100, 200, and 600mg/dL) and 3 different target concentrations of hematocrit (25%, 45%, and 65%). AC data were collected with excitation signals of 10kHz, 2kHz and 1kHz, 9mV rms, and 128Hz, 300mV rms. Then, a DC potential of 550mV was applied. The measurements of this example applied low and high potential AC excitation to the sample.

Data were analyzed using two AC modes as follows:

glucose Int + Yi 1Y 1+ Pi 1P 1+ Yi 2Y 2+ Pi 2P 2+

exp (slope + Ys 1Y 1+ Ps 1P 1+ Ys 2Y 2+ Ps 2P 2) K power (equation 6)

Where 1 is the first AC frequency applied, 2 is the second AC frequency applied, and K may be the K value from equations 4 and 5 or a parameter derived from a 128Hz/300mV measurement (see next paragraph). For simplicity, equation 6 is limited here to two different AC excitations. Equation 6 can be extended to include any number of different AC excitations.

Since this model is designed for increasing values with glucose concentration, it is necessary to derive the parameters from the admittance ratio values used for the above example. This is done according to the following formula:

k3 ═ 90-Arctan (admittance ratio) (equation 7)

In the following analysis, only the AC data is applied, and the value of K3 replaces the value of K in equation 6. AC data was collected for 2.1 seconds, followed by a short open circuit, followed by collection of DC signal data for an additional 2.725 seconds.

Fig. 21 plots the results of applying only the collected DC signal in the analysis. For each measurement sample, the standard error is plotted against the reference glucose level. The error caused by varying sample hematocrit was very recognizable and the results showed that the total error in The System (TSE) was 31.8 mg/dL%.

FIG. 22 plots the results of correcting the DC signal data with the AC data at 10kHz/9mV and 1kHz/9mV using the methods discussed above. By including AC data in the analysis, the total error of the system was significantly reduced to 11.7 mg/dL%.

FIG. 23 plots the results of correcting DC signal data with AC data of 10kHz/9mV and 128Hz/300mV using the methods discussed above. By including the AC data in the analysis, the total error of the system was reduced even further to 5.8 mg/dL%, demonstrating the effectiveness of the 128Hz/300mV data in correcting for DC signal response.

FIG. 24 plots the results using the method discussed above with the K3 parameter (equation 7) derived from 128Hz/300mV data. The hematocrit effect can be recognized, especially at high glucose levels. The total error in the system is 28.4 mg/dL%, which performs similarly to the pure DC signal measurement of FIG. 21.

FIG. 25 plots the results of correcting the K3 data with the AC data at 10kHz/9mV and 1kHz/9mV using the methods discussed above. Note that this is a pure AC test, and only data obtained between 0 and 2.1 seconds are applied in the calculation. The total error of the system is even further reduced to 5.9 mg/dL%.

As shown in the above examples, the systems and methods of the present invention are used for pure AC measurements, in combination with other AC measurement methods, or in combination with other AC and DC measurements, in order to quickly, accurately, and coarsely predict analyte concentrations.

As illustrated in fig. 26, an alternative sensor design 400 was also investigated using the method of the present invention. This design has a single working electrode 402 and two counter electrodes 404 and 406 of the same dimensions that can be contacted individually (although ordinary contact is sufficient) to provide a symmetrical cell for AC measurements. These sensors 400 were tested using the method of the present invention with blood samples ranging from 0 to 520mg/dL and hematocrit ranging from 22% to 65%. Applying DC + low-potential AC at 10kHz and 2kHz and calculating the glucose value with the prior art, the total error of the system tested was 14.9%. Applying the method of the invention and applying 300mV + Low-potential AC at 128Hz AC, 10kHz and 2kHz, the total error of the system is 11%. Applying the method of the invention and applying DC +128Hz 300mVAC + low-potential AC 10kHz, the total error of the system tested was 7.8%. Thus, the electrode structure 400 is also significantly effective for practicing the methods of the present invention.

In any sensor design that employs a pure AC method to determine analyte concentration as described in this disclosure, and in particular in a design with the above-described symmetric cell 400, no electrode can be identified as the working electrode as opposed to the counter electrode, as these terms are generally known to those of ordinary skill in the art of electrochemical biosensors. That is, in a system where a DC signal is applied, when a potential is applied, one of the electrodes becomes an anode and the other becomes a cathode. In an electrooxidation sensor, the analyte is oxidized at the anode and the cathode is the counter electrode. In an electroreductive sensor, the analyte is reduced on the cathode and the anode is the counter electrode. In contrast, for an AC signal lacking a DC offset, the relative potential between the electrodes changes polarity with the period of the applied potential. Thus, the electrode at one point in the cycle is the anode and the electrode at another point in the cycle is the cathode. At the same time, the current response driven by the applied potential induces a potential due to the capacitance of the electrochemical cell. See fig. 1 and 2. Thus, the electrode that is temporarily anodic may draw significant cathodic current, and the electrode that is temporarily cathodic may draw significant anodic current. Furthermore, in the absence of a DC bias potential, there is no net oxidation or reduction of the mediator (or analyte) at either electrode during the measurement. Thus, the measurement can be continued over a considerable period of time without significantly changing the composition of the sample. Repeated measurements may be applied to improve the signal-to-noise ratio of the measurement, to monitor the progress of the enzyme reaction, or to allow the cell to reach a steady state before the final analyte determination is made. As a result, in the sensor of the method of the present invention in which only AC is applied, the electrodes are exchangeable, and the sensor does not have the working electrode and the counter electrode.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.

The following is a list of preferred embodiments of the invention:

1. a method of determining the concentration of a medically significant component of a biological fluid contacted with a reagent compound, comprising the steps of:

a) applying a first signal having an AC component to the biological fluid, wherein the AC component has a magnitude sufficient to generate a Faraday current response from the biological fluid;

b) measuring the current response to the AC component;

c) determining a fundamental component of the current response, the fundamental component comprising a frequency that is at least substantially the same as a frequency of the AC component of the first signal; and

d) determining an indication of the concentration of the medically significant component from the base component.

2. The method of preferred embodiment 1, wherein the first signal is an AC signal.

3. The method of preferred embodiment 1, wherein the galvanic response is caused at least in part by an electrochemical process in the biological fluid.

4. The method of preferred embodiment 1, wherein step (d) comprises determining the indication from a magnitude and a phase angle of the fundamental component.

5. The method of preferred embodiment 1, wherein step (d) comprises determining the indication only from the phase angle of the fundamental component.

6. The method of preferred embodiment 1, wherein the current response comprises an admittance value.

7. The method of preferred embodiment 5, wherein step (d) comprises calculating the tangent of the phase angle of the fundamental component.

8. The method of preferred embodiment 5 wherein the phase angle is calculated relative to a non-zero origin.

9. The method of preferred embodiment 1, wherein the current response is non-linear, and wherein step (c) comprises calculating a first fourier component of the current response.

10. The method of preferred embodiment 9, wherein step (c) comprises calculating a first fourier component of the current response using a transform selected from the group consisting of a fast fourier transform and a discrete fourier transform.

11. The method of preferred embodiment 1, wherein the biological fluid is blood.

12. The method of preferred embodiment 11, wherein the medically significant component is glucose.

13. The method of preferred embodiment 1, wherein the first signal is sinusoidal.

14. The method of preferred embodiment 1, wherein the magnitude of the first signal is between about 200 to 550mV rms.

15. The method of preferred embodiment 1, wherein the first signal has a frequency between about 10 to 1000 Hz.

16. The method of preferred embodiment 1, wherein the first signal has a magnitude of about 300mV rms and a frequency of about 128 Hz.

17. The method of preferred embodiment 1, wherein the first signal has a magnitude of about 40mV rms and a frequency of about 200 Hz.

18. The method of preferred embodiment 1, wherein the concentration of the medically significant component is determined only from the basal component.

19. The method of preferred embodiment 1, further comprising the steps of:

e) prior to said step (a), detecting contact of said biological fluid with said reagent compound, wherein said step (d) is performed within about 4 seconds of said detecting.

20. The method of preferred embodiment 19, wherein said step (d) is performed within about 2 seconds of said detecting.

21. The method of preferred embodiment 20, wherein said step (d) is performed within about 1 second of said detecting.

22. The method of preferred embodiment 1, wherein the first signal further comprises a second AC component having a magnitude insufficient to produce a faraday current response from the biological fluid, and further comprising the steps of:

e) measuring a current response to the second AC component;

f) determining an interferent correction from the current response to the second AC component; and

g) applying the interferent correction to adjust the indication of the concentration from the base component.

23. The method of preferred embodiment 22, further comprising the steps of:

h) prior to said step (a), detecting contact of said biological fluid with said reagent compound, wherein said steps (d) and (g) are performed within about 4 seconds of said detecting.

24. The method of preferred embodiment 23, wherein said step (d) and said step (g) are performed within about 2 seconds of said detecting.

25. The method of preferred embodiment 24, wherein said step (d) and said step (g) are performed within about 1 second of said detecting.

26. The method of preferred embodiment 22 wherein the first signal further comprises a DC component and the method further comprises the steps of:

h) measuring a current response to the DC component;

i) determining an indication of a concentration of the medically significant component from the current response to the DC component; and

j) applying the indication from the fundamental component of the AC component to correct the indication from the DC component, the corrected indication from the DC component to adjust applying the interferer correction.

27. The method of preferred embodiment 1, wherein the first signal further comprises a DC component, and the method further comprises the steps of:

e) measuring a current response to the DC component;

f) determining an indication of a concentration of the medically significant component from the current response to the DC component; and

g) applying the indication from the fundamental component of the AC component to correct the indication from the DC component.

28. The method of preferred embodiment 1 wherein the first signal comprises an AC signal having a single frequency.

29. The method of preferred embodiment 1 wherein the first signal comprises an AC signal and a DC signal.

30. The method of preferred embodiment 1 wherein the first signal comprises an AC signal having a plurality of frequencies.

31. A method of determining the concentration of a medically significant component of a biological fluid contacted with a reagent compound, comprising the steps of:

a) applying a first AC signal to the biological fluid, wherein the first AC signal is of a magnitude sufficient to produce a Faraday current response from the biological fluid;

b) measuring the current response to the first AC signal;

c) determining a fundamental component of the current response, the fundamental component comprising a frequency that is at least substantially the same as a frequency of the first signal; and

32. The method of preferred embodiment 31, wherein the galvanic response is caused at least in part by an electrochemical process in the biological fluid.

33. The method of preferred embodiment 31 wherein step (d) comprises determining the indication from a magnitude and phase angle of the fundamental component.

34. The method of preferred embodiment 31 wherein step (d) comprises determining the indication only from the phase angle of the fundamental component.

35. The method of preferred embodiment 31, wherein the current response comprises an admittance value.

36. The method of preferred embodiment 34, wherein step (d) comprises calculating the tangent of the phase angle of the fundamental component.

37. The method of preferred embodiment 34 wherein the phase angle is calculated relative to a non-zero origin.

38. The method of preferred embodiment 31, wherein the current response is non-linear, and wherein step (c) comprises calculating a first fourier component of the current response.

39. The method of preferred embodiment 38, wherein step (c) comprises calculating a first fourier component of the current response using a transform selected from the group consisting of a fast fourier transform and a discrete fourier transform.

40. The method of preferred embodiment 31, wherein the biological fluid is blood.

41. The method of preferred embodiment 40, wherein the medically significant component is glucose.

42. The method of preferred embodiment 31 wherein the first signal is sinusoidal.

43. The method of preferred embodiment 31, wherein the magnitude of the first signal is between about 200 to 550mV rms.

44. The method of preferred embodiment 31 wherein the first signal has a frequency of between about 10 to 1000 Hz.

45. The method of preferred embodiment 31, wherein the first signal has a magnitude of about 300mV rms and a frequency of about 128 Hz.

46. The method of preferred embodiment 31, wherein the first signal has a magnitude of about 40mV rms and a frequency of about 200 Hz.

47. The method of preferred embodiment 31, wherein the concentration of the medically significant component is determined only from the basal component.

48. The method of preferred embodiment 31, further comprising the steps of:

49. The method of preferred embodiment 48, wherein said step (d) is performed within about 2 seconds of said detecting.

50. The method of preferred embodiment 49, wherein said step (d) is performed within about 1 second of said detecting.

51. The method of preferred embodiment 31, further comprising the steps of:

e) applying a second AC signal to the biological fluid, wherein the second AC signal has a magnitude insufficient to produce a Faraday current response from the biological fluid;

f) measuring a current response to the second AC signal;

g) determining an interferent correction from a phase angle of the current response to the second AC signal; and

h) applying the interferent correction to adjust the indication of the concentration from the base component.

52. The method of preferred embodiment 51, further comprising the steps of:

53. The method of preferred embodiment 52, wherein said step (d) and said step (g) are performed within about 2 seconds of said detecting.

54. The method of preferred embodiment 53, wherein said step (d) and said step (g) are performed within about 1 second of said detecting.

55. The method of preferred embodiment 51, further comprising the steps of:

i) applying a DC signal to the biological fluid;

j) measuring a current response to the DC signal;

k) determining an indication of a concentration of the medically significant component from the current response to the DC signal; and

l) applying the indication from the fundamental component of the first AC signal to correct the indication from the DC signal, the corrected indication from the DC component being adjusted using the interferer correction.

56. The method of preferred embodiment 31, further comprising the steps of:

e) applying a DC signal to the biological fluid;

f) measuring a current response to the DC signal;

g) determining an indication of a concentration of the medically significant component from the current response to the DC signal; and

h) applying the indication from the fundamental component of the first AC signal to correct the indication from the DC signal.

57. The method of preferred embodiment 31 wherein the first AC signal comprises an AC signal having a single frequency.

58. The method of preferred embodiment 31 wherein the first AC signal comprises an AC signal having a plurality of frequencies.

59. A method of determining the glucose concentration of a blood sample contacted with a reagent compound comprising the steps of:

a) applying a first signal having an AC component to said blood sample, wherein said AC component has a magnitude sufficient to produce a faraday current response from said blood sample;

b) measuring the current response to the AC component;

c) determining a fundamental component of the response, the fundamental component comprising a frequency that is at least substantially the same as a frequency of the AC component of the first signal; and

d) determining an indication of the glucose concentration from the base component.

60. The method of preferred embodiment 59, wherein the first signal is an AC signal.

61. The method of preferred embodiment 59, wherein said galvanic response is caused at least in part by an electrochemical process in said blood sample.

62. The method of preferred embodiment 59, wherein step (d) comprises determining the indication from a magnitude and a phase angle of the fundamental component.

63. The method of preferred embodiment 59, wherein step (d) comprises determining the indication only from the phase angle of the fundamental component.

64. The method of preferred embodiment 59, wherein the current response comprises an admittance value.

65. The method of preferred embodiment 62, wherein step (d) comprises calculating the tangent of the phase angle of the fundamental component.

66. The method of preferred embodiment 62 wherein the phase angle is calculated relative to a non-zero origin.

67. The method of preferred embodiment 59, wherein the current response is non-linear, and wherein step (c) comprises calculating a first Fourier component of the current response.

68. The method of preferred embodiment 67, wherein step (c) comprises calculating a first fourier component of the current response using a transform selected from the group consisting of a fast fourier transform and a discrete fourier transform.

69. The method of preferred embodiment 59 wherein the first signal is sinusoidal.

70. The method of preferred embodiment 59, wherein the magnitude of the first signal is between about 200 to 550mV rms.

71. The method of preferred embodiment 59, wherein the first signal has a frequency between about 10 to 1000 Hz.

72. The method of preferred embodiment 59, wherein the first signal has a magnitude of about 300mV rms and a frequency of about 128 Hz.

73. The method of preferred embodiment 59, wherein the first signal has a magnitude of about 40mV rms and has a frequency of about 200 Hz.

74. The method of preferred embodiment 59, wherein the glucose concentration is determined from the basal component only.

75. The method of preferred embodiment 59, further comprising the steps of:

e) detecting said blood in contact with said reagent compound prior to said applying a first signal, wherein said step (d) is performed within about 4 seconds of said detecting.

76. The method of preferred embodiment 75, wherein said step (d) is performed within about 2 seconds of said detecting.

77. The method of preferred embodiment 76, wherein said step (d) is performed within about 1 second of said detecting.

78. The method of preferred embodiment 59, wherein said first signal further comprises a second AC component having a magnitude insufficient to produce a Faraday current response from said blood, and further comprising the steps of:

e) measuring a current response to the second AC component;

79. The method of preferred embodiment 78, further comprising the steps of:

h) prior to said step (a), detecting contact of said blood with said reagent compound, wherein said steps (d) and (g) are performed within about 4 seconds of said detecting.

80. The method of preferred embodiment 79, wherein said step (d) and said step (g) are performed within about 2 seconds of said detecting.

81. The method of preferred embodiment 80, wherein said step (d) and said step (g) are performed within about 1 second of said detecting.

82. The method of preferred embodiment 78, wherein the first signal further comprises a DC component, and the method further comprises the steps of:

h) measuring a current response to the DC component;

i) determining an indication of the glucose concentration from the current response to the DC component; and

83. The method of preferred embodiment 59 wherein the first signal further comprises a DC component, and further comprising the steps of:

e) measuring a current response to the DC component;

f) determining an indication of the glucose concentration from the current response to the DC component; and

84. The method of preferred embodiment 59 wherein the first signal comprises an AC signal having a single frequency.

85. The method of preferred embodiment 59 wherein the first signal comprises an AC signal having a plurality of frequencies.

86. The method of preferred embodiment 59 wherein the first signal comprises an AC signal and a DC signal.

87. A system for determining the concentration of a medically significant component of a biological fluid, the system comprising:

a biosensor comprising at least two electrically insulated electrodes and a reagent compound in proximity to or in contact with at least one of the electrodes;

a measurement device in electrical communication with the electrodes of the biosensor, the device configured and arranged to perform a measurement sequence and data evaluation when the biological fluid is in contact with the at least 2 electrodes and the reagent compound so as to place the electrodes in electrical communication with each other, and between the fluid and the reagent compound;

the measurement sequence comprises:

applying the at least 2 electrodes, applying a first signal having an AC component to the biological fluid, wherein the AC component has a magnitude sufficient to generate a faraday current response from the biological fluid;

measuring the current response to the AC component;

determining a fundamental component of the current response, the fundamental component comprising a frequency that is at least substantially the same as a frequency of the AC component of the first signal; and

determining an indication of the concentration of the medically significant component from the base component.

88. The system of preferred embodiment 87, wherein said determining an indication of said concentration comprises determining said indication from a magnitude and a phase angle of said fundamental component.

89. The system of preferred embodiment 87 wherein the current response comprises an admittance value.

90. The system of preferred embodiment 87 wherein the determining the indication of the concentration comprises calculating a phase angle of the fundamental component, the phase angle calculated relative to a non-zero origin.

91. The system of preferred embodiment 87 wherein the current response is non-linear and wherein said determining the fundamental component of the current response comprises calculating a first fourier component of the current response.

92. The system of preferred embodiment 87, wherein the biological fluid is blood.

93. The system of preferred embodiment 92, wherein the medically significant component is glucose.

94. The system of preferred embodiment 87 wherein the first signal is sinusoidal.

95. The system of preferred embodiment 87 wherein the magnitude of the first signal is between about 200 and 550mV rms.

96. The system of preferred embodiment 87 wherein the first signal has a frequency between about 10 and 1000 Hz.

97. The system of preferred embodiment 87 wherein the first signal has a magnitude of about 300mV rms and a frequency of about 128 Hz.

98. The system of preferred embodiment 87 wherein the first signal has a magnitude of about 40mV rms and a frequency of about 200 Hz.

99. The system of preferred embodiment 87, wherein said measurement sequence further comprises detecting contact of said biological fluid with said reagent compound prior to application of said first signal, wherein said determining an indication of said concentration is performed within about 4 seconds of said detecting.

100. The system of preferred embodiment 99, wherein said determining an indication of said concentration is performed within about 2 seconds of said detecting.

101. The system of preferred embodiment 100, wherein said determining an indication of said concentration is performed within about 1 second of said detecting.

102. The system of preferred embodiment 87, wherein the first signal further comprises a second AC component having a magnitude insufficient to produce a faraday current response from the biological fluid, and the measurement sequence further comprises: measuring a current response to the second AC component, determining an interferent correction from a phase angle of the current response to the second AC component, and applying the interferent correction to adjust an indication of the concentration from the base component.

103. The system of preferred embodiment 102, wherein said measuring sequence further comprises detecting contact of said biological fluid with said reagent compound prior to applying said first signal, wherein said determining an indication of said concentration and said applying said interferent-corrected adjusting said indication occur within about 4 seconds of said detecting.

104. The system of preferred embodiment 103, wherein said determining an indication of said concentration and said applying said interferent correction to adjust said indication are performed within about 2 seconds of said detecting.

105. The system of preferred embodiment 104, wherein said determining an indication of said concentration and said applying said interferent correction to adjust said indication are performed within about 1 second of said detecting.

106. The system of preferred embodiment 102 wherein the first signal further comprises a DC component and the measurement sequence further comprises: measuring a current response to the DC component; determining an indication of a concentration of the medically significant component from the current response to the DC component; and applying the indication from the fundamental component of the AC component to correct the indication from the DC component, the corrected indication from the DC component to adjust using the interferer correction.

107. The system of preferred embodiment 87 wherein the first signal further comprises a DC component and the measurement sequence further comprises: measuring a current response to the DC component; determining an indication of a concentration of the medically significant component from the current response to the DC component; and applying the indication from the fundamental component of the AC component to correct the indication from the DC component.

108. The system of preferred embodiment 87 wherein the first signal comprises an AC signal having a single frequency.

109. The system of preferred embodiment 87 wherein the first signal comprises an AC signal and a DC signal.

110. The system of preferred embodiment 87 wherein the first signal comprises an AC signal having a plurality of frequencies.

111. The method of preferred embodiment 27 wherein the AC component and the DC component are applied sequentially.

112. The method of preferred embodiment 56 wherein the AC component and the DC component are applied sequentially.

113. The method of preferred embodiment 82 wherein the AC component, the second AC component and the DC component are applied sequentially.

114. The method of preferred embodiment 106 wherein the AC component, the second AC component and the DC component are applied sequentially.

115. The method of preferred embodiment 1, further comprising the steps of:

e) prior to said step (a), detecting contact of said biological fluid with said reagent compound; and

f) after said step (d), displaying the concentration of said medically significant component, wherein said step (f) is performed within about 4 seconds of said detecting.

116. The method of preferred embodiment 115, wherein said step (f) is performed within about 2 seconds of said detecting.

117. The method of preferred embodiment 116, wherein said step (f) is performed within about 1 second of said detecting.

118. The method of preferred embodiment 22, further comprising the steps of:

h) prior to said step (a), detecting contact of said biological fluid with said reagent compound; and

i) after said step (g), displaying the concentration of said medically significant component, wherein said step (i) is performed within about 4 seconds of said detecting.

119. The method of preferred embodiment 118, wherein said step (i) is performed within about 2 seconds of said detecting.

120. The method of preferred embodiment 119, wherein said step (i) is performed within about 1 second of said detecting.

121. The method of preferred embodiment 31, further comprising the steps of:

122. The method of preferred embodiment 121, wherein said step (f) is performed within about 2 seconds of said detecting.

123. The method of preferred embodiment 122, wherein said step (f) is performed within about 1 second of said detecting.

124. The method of preferred embodiment 51, further comprising the steps of:

i) prior to said step (a), detecting contact of said biological fluid with said reagent compound; and

j) after said step (h), displaying the concentration of said medically significant component, wherein said step (j) is performed within about 4 seconds of said detecting.

125. The method of preferred embodiment 124, wherein said step (j) is performed within about 2 seconds of said detecting.

126. The method of preferred embodiment 125, wherein said step (j) is performed within about 1 second of said detecting.

127. The method of preferred embodiment 59, further comprising the steps of:

e) prior to said step (a), detecting contact of said blood sample with said reagent compound; and

f) after said step (d), displaying said glucose concentration, wherein said step (f) is performed within about 4 seconds of said detecting.

128. The method of preferred embodiment 127, wherein said step (f) is performed within about 2 seconds of said detecting.

129. The method of preferred embodiment 128, wherein said step (f) is performed within about 1 second of said detecting.

130. The method of preferred embodiment 78, further comprising the steps of:

h) prior to said step (a), detecting contact of said blood sample with said reagent compound; and

i) after said step (g), displaying said glucose concentration, wherein said step (i) is performed within about 4 seconds of said detecting.

131. The method of preferred embodiment 130, wherein said step (i) is performed within about 2 seconds of said detecting.

132. The method of preferred embodiment 131, wherein said step (i) is performed within about 1 second of said detecting.

133. The system of preferred embodiment 87 wherein:

the measurement sequence further comprises detecting contact of the biological fluid with the reagent compound prior to applying the first signal; and

the measurement sequence further comprises displaying a concentration of the medically significant component, wherein the displaying the concentration is performed within about 4 seconds of the detecting.

134. The system of preferred embodiment 133, wherein said displaying said concentration is performed within about 2 seconds of said detecting.

135. The system of preferred embodiment 134 wherein said displaying said concentration is performed within about 1 second of said detecting.

136. The system of preferred embodiment 102 wherein:

the measurement sequence further comprises displaying an adjusted concentration of the medically significant component, wherein the displaying the concentration is performed within about 4 seconds of the detecting.

137. The system of preferred embodiment 136, wherein said displaying said adjusted concentration is performed within about 2 seconds of said detecting.

138. The system of preferred embodiment 137, wherein said displaying said adjusted concentration is performed within about 1 second of said detecting.

Claims

1. A method for determining the concentration of a medically significant component of a biological fluid contacted with a reagent compound, comprising the steps of:

b) measuring the current response to the AC component;

2. The method of claim 1, wherein the first signal is an AC signal.

3. The method of claim 1 or 2, wherein said step (d) comprises determining said indication from a magnitude and/or phase angle of said fundamental component.

4. The method of claims 1-3, wherein the magnitude of the first signal is between about 200-550 mV rms.

5. The method of claims 1-4, wherein the first signal has a frequency of between about 10 to 1000 Hz.

6. The method of claims 1 to 5, wherein the concentration of the medically significant component is determined only from the basal component.

7. The method of claims 1 to 6, wherein the first signal further comprises a second AC component having a magnitude insufficient to produce a Faraday current response from the biological fluid, and further comprising the steps of:

e) measuring a current response to the second AC component;

8. The method of claims 1 to 7, further comprising the steps of:

prior to said step (a), detecting contact of said biological fluid with said reagent compound, wherein said step (d) and/or said step (g) is performed within about 4 seconds of said detecting.

9. The method of claims 1 to 8, wherein the first signal further comprises a DC component, and the method further comprises the steps of:

h) measuring a current response to the DC component;

10. A system for determining a concentration of a medically significant component of a biological fluid, the system comprising:

the measurement sequence comprises:

applying a first signal having an AC component to the biological fluid using the at least 2 electrodes, wherein the AC component has a magnitude sufficient to generate a faraday current response from the biological fluid;

measuring the current response to the AC component;

11. The system of claim 10, wherein the determining the indication of the concentration comprises determining the indication from a magnitude and/or phase angle of the fundamental component.

12. The system of claim 10 or 11, wherein the measurement sequence further comprises detecting contact of the biological fluid with the reagent compound prior to applying the first signal, wherein the determining the indication of the concentration is performed within about 4 seconds of the detecting.

13. The system of claims 10 to 12, wherein the first signal further comprises a second AC component having a magnitude insufficient to produce a faraday current response from the biological fluid, and the measurement sequence further comprises: measuring a current response to the second AC component; determining an interferent correction from a phase angle of the current response to the second AC component; and applying the interferent correction to adjust the indication of the concentration from the base component.

14. The system of claims 10 to 13, wherein the first signal further comprises a DC component and the measurement sequence further comprises: measuring a current response to the DC component; determining an indication of a concentration of the medically significant component from the current response to the DC component; and applying the indication from the fundamental component of the AC component to correct the indication from the DC component, the corrected indication from the DC component to adjust using the interferer correction.