System and method for measuring an analyte in a sample
Methods for calculating an analyte concentration of a sample are provided. In one exemplary embodiment the method includes steps that are directed toward accounting for inaccuracies that occur as a result of temperature variations in a sample, a meter, or the surrounding environment. In another exemplary embodiment the method includes steps that are directed toward determining whether an adequate sample is provided in a meter because insufficient samples can result in inaccuracies. The methods that are provided can be incorporated into a variety of mechanisms, but they are primarily directed toward glucose meters for blood samples and toward meters for controls solutions.
1. A method for determining whether a test strip is sufficiently filled with a sample and distinguishing the sample as a control solution or a physiological sample, the test strip comprising a first electrode and a second electrode, the first electrode being coated with a reagent layer and the second electrode not having a reagent layer coating, and the method comprising:
selecting a first DC test voltage and a first time interval, the first DC test voltage being selected to minimize disturbances of a distribution of a reduced mediator within the test strip, the reduced mediator being a reaction product of the sample and the reagent layer of the first electrode, wherein the first time interval is approximately 1 second;
applying the selected first DC test voltage between the first electrode and the second electrode of the test strip for the first time interval once liquid is detected in the test strip to distinguish between a control solution and a physiological sample;
measuring a capacitance value, the measuring comprising:
applying a second test voltage between the first electrode and the second electrode for a second time interval directly after the first voltage applying step, the second test voltage having a DC voltage component and a superimposed AC voltage component in which the DC voltage component has a larger absolute magnitude than the first DC test voltage, the AC voltage component being applied for only a predetermined time interval starting at a predetermined amount of time after the application of the first DC test voltage and the DC voltage component of the second test voltage, the DC voltage component having a magnitude sufficient to cause a limiting test current at the second electrode, wherein a wetted electrode area of the first electrode increases during the measuring and the second electrode not having the reagent layer coating facilitates measuring the capacitance value notwithstanding the increase of the wetted electrode area during the measuring, and wherein the second time interval is selected to allow sufficient time for distinguishing the sample as the control solution or the physiological sample without interference from the AC voltage and the predetermined amount of time of application of the AC voltage component after the application of the first DC test voltage and the DC voltage component of the second test voltage is selected to allow the sample to fill a sample-receiving chamber of the test strip and the reagent layer to at least partially dissolve before applying the AC voltage component and minimize perturbing a subsequent analyte concentration measurement;
processing a portion of the test currents, resulting from the AC voltage component, into the capacitance value in which a portion of the test currents, resulting from the AC voltage component and the DC voltage component of the second test voltage, are summed only between at about a ¼ wavelength before and about a ¼ wavelength after one of a zero voltage crossing point or a maximum AC voltage component; and
utilizing the measured capacitance value to determine whether the test strip is sufficiently filled with the sample, by determining that the test strip is sufficiently filled with the sample if the measured capacitance value is greater than a predetermined threshold and determining that the test strip is not sufficiently filled with the sample if the capacitance value is less than a predetermined threshold.
2. The method of claim 1 , in which the limiting test current is a limiting oxidation test current where substantially all of a reduced mediator has been depleted at a surface of the second working electrode.
3. The method of claim 1 , in which a magnitude of the second test voltage is sufficient to oxidize a reduced mediator at the second electrode.
4. The method of claim 1 , in which the DC voltage component is applied at a beginning of the second test voltage.
5. The method of claim 1 , in which the DC voltage component is about −0.3 volts with respect to the second electrode.
6. The method of claim 1 , in which the AC voltage component is a sine wave having a frequency of about 109 Hertz and an amplitude of about +/−50 millivolts.
7. The method of claim 1 , in which the reagent layer coating is on the first electrode.
8. The method of claim 7 , in which the reagent layer is configured to generate reduced mediator in a presence of an analyte.
9. The method of claim 8 , in which the reagent layer comprises a mediator and an enzyme where the enzyme does not substantially diffuse from the first electrode to the second electrode when sample is introduced into the test strip.
10. The method of claim 8 , in which the first electrode and the second electrode are in an opposing face format.
11. The method of claim 1 , in which the predetermined amount of time ranges from about 0.3 seconds to about 0.4 seconds after the application of the first test voltage.
12. The method of claim 1 , in which the predetermined amount of time is a time where a test current transient as a function of time has a slope of about zero.
13. The method of claim 1 , in which the predetermined amount of time is a time required for a peak current value to decay by about 50%.
14. The method of claim 1 , in which the predetermined time is not later than 1.4 seconds following the application of the first test voltage.