Hematocrit corrected glucose measurements using phase angles and impedance for electrochemical test strip
Two techniques of determining hematocrit using impedance and phase angle to determine hematocrit are shown and described for correcting an analyte concentration.
1. A method to determine analyte concentration in a blood sample, the method comprising:
depositing a blood sample in a test chamber of a test strip having at least first and second electrodes in contact with a reagent and third and fourth electrodes not in contact with the reagent;
driving a non-oscillating signal to the at least first and second electrodes to cause a reaction with the glucose in the blood sample and the reagent;
measuring a current output of the reaction to establish a preliminary analyte concentration;
applying a first oscillating input signal at a first frequency to third and fourth electrodes, the first frequency comprising any frequency in a range of about 20,000 Hertz to about 60,000 Hertz;
determining a first estimated hematocrit based on an impedance from output signals of the third and fourth electrodes;
applying a second oscillating input signal at a second frequency higher than the first frequency to the third and fourth electrodes, the second frequency comprising any frequency in a range from about 100,000 Hertz to about 1,000,000 Hertz;
ascertaining a second estimated hematocrit based on a phase angle of output signals from the third and fourth electrodes;
estimating a hematocrit of the blood sample based on the first and second estimated hematocrits; and
deriving a final analyte concentration based on the preliminary analyte concentration and the hematocrit from the estimating step.
2. The method of claim 1 , in which the second frequency is one order of magnitude higher than the first frequency.
3. The method of claim 2 , in which the first frequency comprises about 25,000 Hertz and the second frequency comprises about 250,000 Hertz.
4. The method of claim 1 , in which the determining comprises utilizing an equation to determine the first estimated hematocrit HCT est1 :
HCT
est
1
=
Z
-
c
1
m
1
where |Z| comprises the measured modulus of impedance of the blood sample at the first frequency, c1 and m1 are calibration coefficients.
5. The method of claim 4 , in which the measuring of a current output for the preliminary analyte concentration is performed prior to the estimating of the hematocrit.
6. The method of claim 4 , in which the deriving comprises calculating the final analyte concentration G F from:
G
F
=
(
I
E
1
-
k
(
HCT
final_est
-
h
0
)
-
c
4
m
4
.
Where
I E comprises a magnitude of the measured end current
HCT final _ est comprises the hematocrit,
h 0 comprises nominal hematocrit,
and k, m4 and c 4 comprise parameters from regressions data.
7. The method of claim 1 , in which the ascertaining comprises utilizing an equation to determine the second estimated hematocrit HCT est2 of the form:
HCT
est
2
=
(
Φ
-
c
2
)
m
2
where Φ comprises a measured phase angle of the blood sample at one or more second frequencies higher than the first frequency; c2 and m2 comprise respective calibration coefficients.
8. The method of claim 1 , in which the ascertaining comprises utilizing an equation to determine the second estimated hematocrit HCT est2 of the form:
HCT
est
2
=
(
Φ
f
2
-
Φ
f
1
-
c
3
)
m
3
where Φ f2 comprises a measured phase angle of the blood sample at the second frequency; Φ f1 comprises a measured phase angle of the blood sample at the first frequency lower than the second frequency; c3 and m3 each comprises a calibration coefficient.
9. The method of claim 1 , in which the estimating comprises utilizing an equation for a final estimated hematocrit HCT final _ est of the form:
HCT final _ est =a *HCT est1 +( a− 1)*HCT est2
where HCT est1 comprises the first estimated hematocrit and HCT est2 comprises a second estimated hematocrit and a≦1.
10. The method of claim 1 , in which the analyte comprises glucose.