METHOD FOR DETERMINING HEMATOCRIT CORRECTED ANALYTE CONCENTRATIONS

• US 20100219084A1
• Filed: 10/05/2007
• Published: 09/02/2010
• Est. Priority Date: 10/05/2006
• Status: Active Grant

• Current Assignee: Cilag Gmbh International (Johnson & Johnson)
• Original Assignee: LifeScan Scotland Limited (Johnson & Johnson)
• Inventors: Leach, Christopher Philip, Cardosi, Marco F., Mills, Leanne, Gill, Andrew, Blythe, Stephen Patrick
• Granted Patent: US 8,388,821 B2
• Time in Patent Office: Days
• US Class Current: 205/777.500
• CPC Class Codes: C12Q 1/006 for glucose
• CPC Class Codes: G01N 27/3271 Amperometric enzyme electrodes for analytes in body fluids, e.g. glucose in blood amperometry per se G01N27/49; aspects concerning the enzyme reagent C12Q1/001

Abstract

Description is provided herein for an embodiment of a method determining a hematocrit-corrected glucose concentration. The exemplary method includes providing a test strip having a reference electrode and a working electrode, wherein the working electrode includes a plurality of microelectrodes and is coated with at least an enzyme and a mediator. The method can be achieved by: providing a test strip comprising a reference electrode and a working electrode formed with a plurality of microelectrodes and coated with a reagent layer; applying a fluid sample to the test strip for a reaction period; applying a test voltage between the reference electrode and the working electrode; measuring a test current as a function of time; measuring a steady state current value when the test current has reached an equilibrium; calculating a ratio of the test current to the steady state current value; plotting the ratio of the test current to the steady state current value as a function of the inverse square root of time; calculating an effective diffusion coefficient from the slope of the linearly regressed plot of the ratio of the test current to the steady state current value as a function of the inverse square root of time; and calculating a hematocrit-corrected concentration of analyte.

Claims

1. A method of calculating a hematocrit-corrected glucose concentration in a fluid sample, the method comprising:

providing a test strip comprising a reference electrode and a working electrode formed with a plurality of microelectrodes and coated with a reagent layer;

applying a fluid sample to the test strip for a reaction period;

applying a test voltage between the reference electrode and the working electrode;

measuring a test current as a function of time;

measuring a steady state current value when the test current has reached an equilibrium;

calculating a ratio of the test current to the steady state current value;

plotting the ratio of the test current to the steady state current value as a function of the inverse square root of time;

calculating an effective diffusion coefficient from the slope of the linearly regressed plot of the ratio of the test current to the steady state current value as a function of the inverse square root of time; and

calculating a hematocrit-corrected concentration of analyte.

2. The method of claim 1, wherein the calculating of the effective diffusion coefficient step utilizes an equation of the form:

$\frac{I<br><br>\left(t\right)}{I_{\mathrm{SS}}}=1+\left(\frac{2<br><br>r_d}{\mathrm{\pi <br><br>}<br><br>\sqrt{\mathrm{\pi <br><br>}<br><br><br><br>\mathrm{Dt}}}\right)$

where;

I(t) is the current value in microamperes measured at time t;

I_SS is the steady state current in microamperes;

r_d is the radius of a microelectode in centimeters; and

t is time in seconds.

3. The method of claim 2, wherein the calculating a hematocrit-corrected concentration of analyte step further comprises:

substituting the estimated diffusion coefficient into an equation of the form;

$C_{\mathrm{red}}=\frac{I_S}{4<br><br>{\mathrm{nFDr}}_d}$

where;

C_red is a reduced mediator concentration;

I_SS is the steady state current in microamperes;

n is the number of electrons exchanged per ion that undergoes an oxidation/reduction reaction;

F is Faraday'"'"'s constant;

D is the estimated diffusion coefficient in centimeter²/second; and

r_d is the radius of the microelectrode in centimeters;

generating a calibration curve in which a y-axis is the reduced mediator concentration and an x-axis is a reference analyte concentration;

calculating an analyte concentration by subtracting a calibration intercept from the reduced mediator concentration and dividing with a calibration slope.

4. The method of claim 1, wherein the test strip further comprises an insulation portion disposed on the plurality of microelectrodes and the insulation portion has a height between about one micron and about six microns.

5. The method of claim 1, wherein the reference electrode and the working electrode are comprised of gold.

6. The method of claim 1, wherein the reagent layer comprises an enzyme, a mediator and a buffering agent wherein the mediator comprises ruthenium (III) hexamine.

7. The method of claim 1, wherein the diameter of each of the plurality of microelectrodes is between about 5 microns and about 50 microns.

8. The method of claim 1, wherein each of the plurality of microelectrodes is spaced apart by a distance ranging from about 5 to about 10 times the diameter of a microelectrode.

9. The method of claim 1, wherein each of the plurality of microelectrodes is spaced apart by a distance ranging from about 25 microns to about 500 microns.

10. The method of claim 1, wherein the shape of each of the plurality of microelectrodes is selected from a group consisting essentially of a circle, a square, a rectangle, an oval and an ellipse and combinations thereof.

11. A method of determining a type of fluid sample applied to the test strip, the method comprising:

providing a test strip having a reference electrode and a working electrode, wherein the working electrode is formed with a plurality of microelectrodes and is coated with a reagent layer;

applying a fluid sample to the test strip for a reaction period;

applying a test voltage between the reference electrode and the working electrode;

measuring a test current as a function of time;

measuring a steady state current value when the test current has reached an equilibrium;

calculating a ratio of the test current to the steady state current value;

plotting the ratio of the test current to the steady state current value as a function of the inverse square root of time;

calculating an effective diffusion coefficient from the slope of the linearly regressed plot of the ratio of the test current to the steady state current value as a function of the inverse square root of time;

determining a type of a fluid sample applied to the test strip by comparing a measured value for the effective diffusion coefficient against an acceptance range for a bodily fluid and a control solution; and

displaying an appropriate error message if the effective diffusion coefficient does not pass the acceptance range for the type of fluid sample applied to the test strip.

12. The method of claim 11, wherein the calculating of the effective diffusion coefficient step utilizes an equation of the form:

$\frac{I<br><br>\left(t\right)}{I_{\mathrm{SS}}}=1+\left(\frac{2<br><br>r_d}{\mathrm{\pi <br><br>}<br><br>\sqrt{\mathrm{\pi <br><br>}<br><br><br><br>\mathrm{Dt}}}\right)$

where;

I(t) is the current value in microamperes measured at time t;

I_SS is the steady state current in microamperes;

r_d is the radius of a microelectode in centimeters; and

t is time in seconds.

13. The method of claim 12, wherein the calculating a hematocrit-corrected concentration of analyte step further comprises:

substituting the estimated diffusion coefficient into an equation of the form;

$C_{\mathrm{red}}=\frac{I_S}{4<br><br>{\mathrm{nFDr}}_d}$

where;

C_red is a reduced mediator concentration;

I_SS is the steady state current in microamperes;

n is the number of electrons exchanged per ion that undergoes an oxidation/reduction reaction;

F is Faraday'"'"'s constant;

D is the estimated diffusion coefficient in centimeter²/second; and

r_d is the radius of the microelectrode in centimeters;

generating a calibration curve in which a y-axis is the reduced mediator concentration and an x-axis is a reference analyte concentration;

calculating an analyte concentration by subtracting a calibration intercept from the reduced mediator concentration and dividing with a calibration slope.

14. The method of claim 11, wherein the test strip further comprises an insulation portion disposed on the plurality of microelectrodes and the insulation portion has a height between about one micron and about six microns.

15. The method of claim 11, wherein the reference electrode and the working electrode are comprised of gold.

16. The method of claim 11, wherein the reagent layer comprises an enzyme, a mediator and a buffering agent wherein the mediator comprises ruthenium (III) hexamine.

17. The method of claim 11, wherein the diameter of each of the plurality of microelectrodes is between about 5 microns and about 50 microns.

18. The method of claim 11, wherein each of the plurality of microelectrodes is spaced apart by a distance ranging from about 5 to about 10 times the diameter of a microelectrode.

19. The method of claim 11, wherein each of the plurality of microelectrodes is spaced apart by a distance ranging from about 25 microns to about 500 microns.

20. The method of claim 11, wherein the shape of each of the plurality of microelectrodes is selected from a group consisting of a circle, a square, a rectangle, an oval and an ellipse.

21. A system comprising:

a test strip having a reference electrode and a working electrode, wherein the working electrode formed with a plurality of microelectrodes and coated with an reagent layer; and

a test meter comprising;

an electronic circuit for applying a test voltage between the reference electrode and the working electrode; and

a signal processor for measuring a test current and for calculating an effective diffusion coefficient.

22. The system of claim 21, wherein the test strip further comprises an insulation portion disposed on the plurality of microelectrodes and the insulation portion has a height between about one micron and about six microns.

23. The system of claim 21, wherein the effective diffusion coefficient is calculated from the slope of the linearly regressed plot of

$\frac{I}{}$

( t ) I SS

versus

$\frac{1}{\sqrt{t}}$

based on an equation of the form;

$\frac{I<br><br>\left(t\right)}{I_{\mathrm{SS}}}=1+\left(\frac{2<br><br>r_d}{\mathrm{\pi <br><br>}<br><br>\sqrt{\mathrm{\pi <br><br>}<br><br><br><br>\mathrm{Dt}}}\right)$

where;

I(t) is the test current in microamperes measured at time t;

I_SS is a steady state current in microamperes;

r_d is a radius of a microelectode in centimeters; and

t is time in seconds.

24. The system of claim 23, wherein a hematocrit-corrected glucose concentration is calculated by using the estimated diffusion coefficient and an equation of the form:

$C_{\mathrm{red}}=\frac{I_S}{4<br><br>{\mathrm{nFDr}}_d}$

where;

C_red is a reduced mediator concentration;

I_SS is the steady state current in microamperes;

n is the number of electrons exchanged per ion that undergoes an oxidation/reduction reaction;

F is Faraday'"'"'s constant;

D is a estimated diffusion coefficient in centimeter²/second; and

r_d is a radius of the microelectrode in centimeters.

25. The system of claim 34, wherein an effective diffusion coefficient is used to discriminate between a fluid sample of whole blood and a fluid sample of control solution by comparing the effective diffusion coefficient to an acceptance range for whole blood and an acceptance range for control solution and the effective diffusion coefficient is used to determine if a test strip is formed with a plurality of microelectrodes by substituting the effective diffusion coefficient into an equation of the form:

$\tilde{D}=D<br><br>\exp <br><br>\left\{\mathrm{\theta <br><br>}<br><br>\left(\frac{1}{T}-\frac{1}{T_0}\right)\right\}$

Where;

{tilde over (D)} is a temperature-corrected effective diffusion coefficient in centimeter²/second;

D is a estimated effective diffusion coefficient in centimeter²/second;

θ

is a known constant for temperature-dependent diffusion;

T is a temperature in Kelvin of the fluid sample as measured by the test meter; and

T₀ is a reference temperature in degrees Kelvin.