CALIBRATION CONCEPT FOR AMPEROMETRIC CREATININE SENSOR CORRECTING FOR ENDOGENOUS MODULATORS

Abstract

A method of calibrating a device for measuring the concentration of creatinine in a sample including one or more enzyme modulators, the method comprising: determining sensitivities of the device for each of two or more calibration solutions, wherein each calibration solution has a different amount of enzyme modulator; determining a degree of modulation for each of the two or more calibration solutions; determining a degree of modulation for a sample to be measured; and calculating the sensitivity of the device for the sample, wherein said calculating comprises modifying the sensitivity of one of the two or more calibration solutions by a function comprising the determined degrees of modulation.

Claims

1. A method for calibrating a device for measuring the concentration of creatinine in a sample comprising one or more enzyme modulators, the method comprising: determining sensitivities of the device for each of two or more calibration solutions, wherein each calibration solution has a different amount of enzyme modulator; determining a degree of modulation for each of the two or more calibration solutions; determining a degree of modulation for a sample to be measured; and calculating the sensitivity of the device for the sample, wherein [[the]] said calculating comprises adjusting the sensitivity of the device for one of the two or more calibration solutions by a factor, wherein factor is a function of the determined degrees of modulation.

2. The method of claim 1, wherein the one or more enzyme modulators include an acid or an alkali or a salt thereof.

3. The method of claim 1, wherein the one or more enzyme modulators comprise one or more of bicarbonate, acetate, formate, Ca2+, and Zn2+, and pH.

4. The method of claim 1, wherein said determining the sensitivities of the device for each of two or more calibration solutions comprises calculating a ratio between an output of the device in the calibration solution and a concentration of creatinine and/or creatine in the calibration solution.

5. The method of claim 1, wherein said determining a degree of modulation for each of the calibration solutions comprises estimating enzyme modulation based on the amounts of enzyme modulators in the calibration solutions.

6. The method of claim 1, wherein said determining a degree of modulation for each of the calibration solutions comprises receiving a value of said degree of modulation.

7. The method of claim 1, wherein the device further comprises a membrane covering the device and facing the sample and said function further comprises a ratio between enzyme activity and permeability of the membrane.

8. The method of claim 7, wherein the ratio between enzyme activity and permeability of one or more membrane layers covering the electrode is a dimensionless constant specific to the device.

9. The method of claim 1, wherein one of the two or more calibration solutions has an amount of enzyme modulator of the same level as an amount of enzyme modulator in the sample.

10. The method of claim 1, wherein one of the two or more calibration solutions has a low level of enzyme modulation and another calibration solution has a higher level of enzyme modulation.

11. The method of claim 1, wherein one of the two or more calibration solutions has a high level of enzyme modulation.

12. The method of claim 1, wherein the device is a creatine and/or creatinine sensor.

13. A computer readable medium comprising instructions which when executed by one or more processors of an electronic device, cause the electronic device to operate in accordance with the method as claimed in claim 1.

14. An electronic device comprising: one or more processors; and memory comprising instructions which when executed by one or more of the one or more processors cause the electronic device to operate in accordance with the method claimed in claim 1.

15. A package containing: two or more calibration solutions, wherein each calibration solution has a different amount of enzyme modulator; and instructions for use with the method of claim 1.

16. A package containing: two or more calibration solutions, wherein each calibration solution has a different amount of enzyme modulator; and instructions for use with the electronic device of claim 14.

Description

BRIEF DESCRIPTIONS OF DRAWINGS

[0025] Examples of the present proposed apparatus will now be described in detail with reference to the accompanying drawings, in which:

[0026] FIG. 1 is a schematic diagram of an example of an amperometric measuring system;

[0027] FIG. 2 is a series of diagrams illustrating the enzyme cascade for conversion of creatinine to hydrogen peroxide;

[0028] FIG. 3 is a table showing examples of enzyme modulators; and

[0029] FIG. 4 is a flowchart outlining the steps of the proposed method.

DETAILED DISCLOSURE

[0030] Reference will now be made to FIG. 1 which is a schematic diagram of a three electrode amperometric measuring system 101. An amperometric measuring system may have at least two electrodes: a working electrode (WE) 110 and a combined counter and reference electrode (CE/RE). For the three-electrode amperometric measuring system 101, the functions of the CE/RE electrode are split into two separate electrodes: the reference electrode (RE) 111 and the counter electrode (CE) 112. The example amperometric measuring system 101 also includes an ammeter 120, a voltmeter 121 and a voltage source 122 and the electrolyte solution 140.

[0031] The WE 110 is a positively charged electrode where an oxidation reaction occurs. The RE 111 is typically made of Ag/AgCl and is able to maintain a stable potential, especially if no current runs through it, thus the need for a CE 112 for passing the current from the WE 110 back to the electrolyte solution 140. The electrolyte solution 140 and the sample 150 provide ionic contact between the three electrodes. The membrane 130 selectively converts the analyte to a substance that selectively is allowed to pass through from the sample 150. The voltage source 122 applies the necessary potential for maintaining the desired reduction or oxidation reaction, this is controlled by the voltmeter 121. The ammeter 120 measures the resulting current flowing through the electrical circuit, which is a measure of the free flowing electrons due to the chemical reactions between the sample 150 and the electrolyte solution 140.

[0032] The amperometric measuring system shown in FIG. 1 is an illustrative example, and several other implementations are envisioned. For example, the amperometric measuring system could be a two electrode system as mentioned above.

[0033] The magnitude of an electrical current flowing through the electrode chain is proportional to the concentration of the substance being oxidized (or reduced) at the WE 110. Ideally, when knowing the proportionality constant relating the electrical current to a concentration, the concentration in any given sample can be obtained by measuring the electrical current generated by that particular sample.

[0034] To illustrate the measuring process in an amperometric measuring system, we assume that: The sample 150 contains species B, which in the membrane 130 is selectively converted to species A, which can be oxidized at the WE 110 (WE) to A.sup.+; and the electrolyte 140 contains species X which is reduced at the CE 112 (cathode) to X.sup.−. We assume also that the membrane 130 allows only species A to pass from the sample into the electrolyte solution 140.

[0035] As an appropriate potential is applied across the electrodes, A is oxidized at the WE 110 according to the following reaction:

A.fwdarw.A.sup.−+e.sup.−

[0036] The oxidation of A produces a flow of electrons. To complete the electrical circuit a reduction reaction where electrons are consumed is necessary. Therefore species X is reduced at the CE 112 according to the following reaction:

X+e.fwdarw.X

[0037] The magnitude of the current flowing through the circuit is proportional to the concentration of the analyte being oxidized. The analyser can therefore automatically calculate the concentration of the analyte in the sample given species X is in excess.

[0038] The term sensor refers to a complete amperometric measuring system, as shown in FIG. 1 excluding the sample 150.

[0039] Crn is not stable in aqueous solutions, e.g. blood, where it is reversibly converted into Cr (see Scheme 1). To measure cCr, a Creatine sensor is used (Crea A).

[0040] To measure cCrn, a two-sensor system can be used where one sensor (Crea A) detects Cr only, and the other sensor (Crea B) detects both Cr and Crn. By means of a difference measurement it is possible to obtain the cCrn value.

##STR00001##

[0041] The sensor is protected by a multilayer membrane 130 consisting of at least three functional layers, namely the outer membrane layer permeable to Crn and Cr; the middle enzyme layer, and the inner membrane layer permeable to H.sub.2O.sub.2.

[0042] In another embodiment, cCrn is determined directly with a sensor that essentially only has a sensitivity towards Crn. This may be done by applying an outer membrane that is impermeable towards Cr but permeable to Crn, which is feasible since Cr is an anion and Crn is neutral.

[0043] FIG. 2 illustrates an example enzyme cascade for the conversion of creatine and creatinine into hydrogen peroxide. In this example, enzymes creatinase (creatine amidinohydrolase) 220, sarcosine oxidase 230 and creatininase (creatinine amidohydrolase) 210 are used in the enzyme cascade. These enzymes are immobilized between the inner and outer membrane layers, while Crn and Cr molecules can diffuse across the outer membrane layer.

[0044] The Crea A sensor detects creatine by converting creatine to hydrogen peroxide in accordance with reactions 202 and 203. To achieve this conversion, the Crea A sensor uses creatine amidinohydrolase 220 and sarcosine oxidase 230. In the Crea A sensor, the enzymatic cascade changes Cr as follows:

Creatine+H.sub.2O.fwdarw.sarcosine+urea (creatine amidinohydrolase)

Sarcosine+H.sub.2O+O.sub.2.fwdarw.glycine+formaldehyde+H.sub.2O.sub.2 (sarcosine oxidase)

[0045] The Crea B sensor contains all three enzymes creatinine amidohydrolase 210, creatine amidinohydrolase 220 and sarcosine oxidase 230, and so detects both Crn and Cr. In the enzymatic cascade Crn/Cr involves reactions 201, 202 and 203:

Creatinine+H.sub.2O⇄creatine (creatinine amidohydrolase)

Creatine+H.sub.2O.fwdarw.sarcosine+urea (creatine amidinohydrolase)

Sarcosine+H.sub.2O+O.sub.2.fwdarw.glycine+formaldehyde+H.sub.2O.sub.2 (sarcosine oxidase)

[0046] For both the Crea A and the Crea B sensors the enzyme reactions lead to identical end-products, one of which is H.sub.2O.sub.2 that can diffuse across the inner membrane layer to the WE 110 (preferably platinum). By applying a sufficiently high electrical potential to the electrode chains of the Crea A and Crea B sensors, H.sub.2O.sub.2 can be oxidized at the Pt anode 240:

H.sub.2O.sub.2.fwdarw.2H.sup.++O.sub.2 +2e

[0047] To complete the electrical circuit, electrons are consumed in reduction reactions at the CE 112 thereby maintaining a charge balance between the WE 110 and the CE 112.

[0048] The oxidation of H.sub.2O.sub.2 produces an electrical current (dE) proportional to the amount of H.sub.2O.sub.2, which in turn is directly related to the amount of Cr for the Crea A and the amount of Cr and Crn for the Crea B sensors according to the sensor response models:

dE.sub.A=Sens.sub.A,Cr.[Cr].sub.A   Equation 1

dE.sub.B=Sens.sub.B,Crn [Crn].sub.B+Sens.sub.B,Cr.Math.[Cr].sub.B   Equation 2

[0049] Where dE.sub.A and dE.sub.B are the electrical currents produced at the Crea A and Crea B sensors respectively; Sens.sub.A,Cr and Sens.sub.B,Cr are the sensitivity constants relating current (dE) to Cr concentration in the Crea A and Crea B sensors respectively and Sens.sub.B,Crn is the sensitivity constant relating current (dE) to Crn concentration in the Crea B sensor.

[0050] The proportionality constants, Sens, relating currents to concentrations are typically referred to as sensitivities. The constants are determined by calibrating the sensors. The current (signal) of each sensor is measured by ammeters 120 in the analyser. If sensor sensitivities S are known, the unknown Crn concentration in a given sample is readily determined from the equations above.

[0051] The reactions illustrated in FIG. 2 can be modulated by enzyme modulators. Such enzyme modulators may be endogenous to the sample, such as bicarbonates, and these enzyme modulators may inhibit the action of any of the enzymes used. The term enzyme modulator includes substances that reduce the performance of enzymes (inhibitors) or increase the performance of the enzymes.

[0052] FIG. 3 shows a number of examples of endogenous modulators that may be present in the sample. The table shows the example modulators along with a measure of their effectiveness in modulating enzymes, namely the I.sub.50 value (half maximal inhibitory concentration) in mM. Enzyme modulators are not limited to specific molecules, and may include other factors such as the pH or temperature of a solution or sample. It is known that factors like the pH of a solution can affect the performance of the enzyme, so factors such as pH may be referred to herein as enzyme modulators.

[0053] In an example embodiment, calibration solutions are set up to determine the effect of pH and bicarbonate concentrations on the sensor readings for a Cr sensor. In this example embodiment, the concentration of Cr is measured using the Crea A sensor, but it is envisioned that the solution may be adapted to measure the concentration of Crn using Crea A and Crea B sensors.

[0054] In this example embodiment, the sensor is calibrated using two calibration solutions denoted Cal2 and Cal3. The following scheme shows the content of the two solutions and a given sample:

TABLE-US-00001 TABLE 1 Cal2 Cal3 Sample cCr [μM] 943 487 TBD pH 6.8 7.1 7.4 [HCO.sub.3] [mM] 0 24 53.2 pCO.sub.2 [mmHmg] 0 80 82.9 dESample (readout) [pA] 31179 14235 2640 Sensitivity ([Cr]/dESample) [pA/μM] 33.1 29.2 TBD

[0055] Table 1 shows how the calibration solutions Cal2 and Cal3 have known concentrations of Cr and HCO.sub.3.sup.−, known pH levels, and known CO.sub.2 partial pressures. Calibrators Cal2 and Cal3 may also contain buffers, salts, preservatives and detergents, but in this example embodiment those will be ignored.

[0056] The sample, which may be a blood sample to be measured, and may have its pH, HCO.sub.3.sup.− concentration and CO.sub.2 partial pressure measured by appropriate sensors. A raw reading (dESample) of the sample can be measured using the amperometric measuring device, but the sensitivity needs to be determined before a Cr concentration can be determined for the sample.

[0057] As the Cr concentrations of Cal2 and Cal3 are known, the sensitivities of the sensor for the calibration solutions can be calculated by measuring the ratio between the measured device output (dE) and the known concentrations:

[00001]$\begin{array}{cc}{\mathrm{sens}}_{\mathrm{Cal}.Math..Math.2}=\frac{\mathrm{dECal}.Math..Math.2}{{\left[\mathrm{Cr}\right]}_{\mathrm{Cal}.Math..Math.2}}=\frac{31179.Math..Math.\mathrm{pA}}{943.Math..Math.\mathrm{.Math.M}}& \mathrm{Equation}.Math..Math.3\\ {\mathrm{sens}}_{\mathrm{Cal}.Math..Math.3}=\frac{\mathrm{dECal}.Math..Math.3}{{\left[\mathrm{Cr}\right]}_{\mathrm{Cal}.Math..Math.3}}=\frac{14235.Math..Math.\mathrm{pA}}{487.Math..Math.\mathrm{.Math.M}}& \mathrm{Equation}.Math..Math.4\end{array}$

[0058] The readout for each calibration solution (dECal) is recorded and transformed into a sensitivity ratio:

[00002]$\begin{array}{cc}\frac{{\mathrm{sens}}_{\mathrm{Cal}.Math..Math.3,2.Math.\mathrm{enz}}}{{\mathrm{sens}}_{\mathrm{Cal}.Math..Math.2,2.Math.\mathrm{enz}}}=\frac{\frac{\mathrm{dECal}.Math..Math.3}{{\left[\mathrm{Cr}\right]}_{\mathrm{Cal}.Math..Math.3}}}{\frac{\mathrm{dECal}.Math..Math.2}{{\left[\mathrm{Cr}\right]}_{\mathrm{Cal}.Math..Math.2}}}=\frac{\frac{14235}{487.Math..Math.\mathrm{.Math.M}}}{\frac{31179.Math..Math.\mathrm{pA}}{943.Math..Math.\mathrm{.Math.M}}}=0.883& \mathrm{Equation}.Math..Math.5\end{array}$

[0059] The ratio 0.883 illustrates that the sensor gives 12.3% less current per μM creatine in a solution like Cal3 than in the bicarbonate free solution Cal2. The degree of modulation (mod) is calculated for both calibrations solutions using the known pH (pH_Cal2 and pH_Cal3), and [HCO.sub.3.sup.−] ([HCO.sub.3].sub.cal2 and [HCO.sub.3.sup.−].sub.cal3):

[00003]$\begin{array}{cc}{\mathrm{mod}}_{\mathrm{Cal}.Math..Math.2}=\frac{\mathrm{Ka}}{{10}^{-\mathrm{pH\_Cal}.Math..Math.2}+\mathrm{Ka}}.Math.\frac{1}{\left(1+{\left[{\mathrm{HCO}}_3^{-}\right]}_{\mathrm{Cal}.Math..Math.2}/K_i\right)}=0.137& \mathrm{Equation}.Math..Math.6\\ {\mathrm{mod}}_{\mathrm{Cal}.Math..Math.3}=\frac{\mathrm{Ka}}{{10}^{-\mathrm{pH\_Cal}.Math..Math.3_{+\mathrm{Ka}}}}.Math.\frac{1}{\left(1+{\left[{\mathrm{HCO}}_3^{-}\right]}_{\mathrm{Cal}.Math..Math.3}/K_i\right)}=0.025& \mathrm{Equation}.Math..Math.7\end{array}$

[0060] Equation 6 and 7 may be derived by skilled person by multiplying the equation for protolyzation of a single base and the expression of simple competitive inhibition.

[0061] The degree of modulation (mod.sub.cal2 and mod.sub.cal3) provides an estimate for how much the enzyme is modulated in the given pH and bicarbonate. K.sub.a is the acid ionization constant for the calibration solution, while K.sub.i is the inhibition constant. For the given example 86.3% (1-0.137) and 97.5% (1-0.025) of the original enzyme activity is expected to be removed by inhibition in Cal2 and Cal3, respectively.

[0062] From the known modulation values and the sensitivity ratios, one can calculate a sensor specific constant phi (φ). Phi is a dimensionless constant that is an expression of the ratio between enzyme activity and the permeability of the sensor. In the example embodiment provided, the value for phi is given by:

[00004]$\begin{array}{cc}\varphi =\frac{1}{{\mathrm{mod}}_{\mathrm{Cal}.Math..Math.3}\left(\frac{{\mathrm{sens}}_{\mathrm{Cal}.Math..Math.2}}{{\mathrm{sens}}_{\mathrm{Cal}.Math..Math.3}}-1\right)}+\frac{1}{{\mathrm{mod}}_{\mathrm{Cal}.Math..Math.2}\left(\frac{{\mathrm{sens}}_{\mathrm{Cal}.Math..Math.3}}{{\mathrm{sens}}_{\mathrm{Cal}.Math..Math.2}}-1\right)}=238.3& \mathrm{Equation}.Math..Math.8\end{array}$

[0063] Once a sample is aspirated, the modulation of the sensor with the pH and bicarbonate from this specific sample can be calculated:

[00005]$.Math.\mathrm{Equation}.Math..Math.9$$\begin{array}{c}\mathrm{mod}\left({\left[\mathrm{pH}\right]}_{\mathrm{enz}},{\left[{\mathrm{HCO}}_3^{-}\right]}_{\mathrm{Enz}}\right)=.Math.\frac{\mathrm{Ka}}{\begin{array}{c}{10}^{-p.Math..Math.H_{\mathrm{rinse}}}+\left({10}^{-p.Math..Math.H_{\mathrm{sample}}}-{10}^{-p.Math..Math.H_{\mathrm{rinse}}}\right)\\ C_3\left(.Math.{p.Math.\mathrm{CO}}_{2_{\mathrm{Sam}}}-{p.Math.\mathrm{CO}}_{2_{\mathrm{Rinse}}}.Math.\right)+\mathrm{Ka}\end{array}}\\ .Math.\frac{1}{\left(1+{\left[{\mathrm{HCO}}_3^{-}\right]}_{\mathrm{enz}}/K_i\right)}\\ =.Math.0.015\end{array}$

[0064] Equation 9 is a rewriting of equation 6 and 7, where an extra term is added that accounts for the effect of pCO.sub.2 on the pH in the sensor.

[0065] Here the pH.sub.rinse value is the pH is the rinse solution the sensor is exposed to between samples, pCO.sub.2rinse is the partial pressure of CO.sub.2 in a rinse solution the sensor is exposed to between samples, and C.sub.3 is a constant fixed for all sensors and correlates to the permeability of CO.sub.2.

[0066] The sensitivity in the given sample is calculated by adjusting the sensitivity of one of the calibration solutions by a factor. The factor is a function of the degree of modulation of the sample and the calibration solution, and may also include the value phi. In this example, the sensitivity for Cal3 (sens.sub.Cal3) is adjusted because the degree of modulation in Cal3 is closer to the degree of modulation of the sample than in Cal2. In this example, the factor is given by:

[00006]$\begin{array}{cc}\frac{1-\frac{1}{\varphi .Math..Math.{\mathrm{mod}}_{\mathrm{Sam}}+1}}{1-\frac{1}{\varphi .Math..Math.{\mathrm{mod}}_{\mathrm{Cal}.Math..Math.3}+1}}& \mathrm{Equation}.Math..Math.10\end{array}$

[0067] In this example the adjusting factor is roughly 0.89, and therefore the sensitivity for the sample is equal to the sensitivity for Cal3 multiplied by this factor:

[00007]$\begin{array}{cc}{\mathrm{sens}}_{\mathrm{Sam},\mathrm{Cr}}={\mathrm{sens}}_{\mathrm{Cal}.Math..Math.3}.Math.\frac{1-\frac{1}{\varphi .Math..Math.{\mathrm{mod}}_{\mathrm{Sam}}+1}}{1-\frac{1}{\varphi .Math..Math.{\mathrm{mod}}_{\mathrm{Cal}.Math..Math.3}+1}}=26.2.Math..Math.\mathrm{pA}.Math.\text{/}.Math.\mathrm{.Math.M}& \mathrm{Equation}.Math..Math.11\end{array}$

[0068] Using the readout on the given sample (e.g. dESamp=2640 pA) gives the corrected creatine content:

[00008]$\begin{array}{cc}{\left[\mathrm{Cr}\right]}_{\mathrm{Sam}}=\frac{\mathrm{dESamp}}{{\mathrm{sens}}_{\mathrm{Sam},\mathrm{Cr},}}=\frac{2640.Math..Math.\mathrm{pA}}{26.2.Math..Math.\mathrm{pA}.Math.\text{/}.Math.\mathrm{.Math.M}}=100.8.Math..Math.\mathrm{.Math.M}& \mathrm{Equation}.Math..Math.12\end{array}$

[0069] If the concentration had been calculated using the sensitivity of Cal3 instead of the sensitivity taking into account enzyme modulators, the concentration found would be about 10% lower:

[00009]$\begin{array}{cc}{\left[\mathrm{Cr}\right]}_{\mathrm{Sam}}=\frac{\mathrm{dESamp}}{{\mathrm{sens}}_{\mathrm{Cal}.Math..Math.3}}=\frac{2640.Math..Math.\mathrm{pA}}{28.8.Math..Math.\mathrm{pA}.Math.\text{/}.Math.\mathrm{.Math.M}}=91.7.Math..Math.\mathrm{.Math.M}& \mathrm{Equation}.Math..Math.13\end{array}$

[0070] Furthermore, if the concentration was calculated using the sensitivity of Cal2, which does not have any bicarbonate enzyme modulators, the concentration of Cr would be 79.6 μM, which differs greatly from the concentration calculated using the proposed method. This illustrates that the proposed solution provides improved results over existing methods that do not take into account enzyme modulators when calibrating sensors using calibration solutions.

[0071] FIG. 4 outlines the steps for carrying out an example embodiment of the proposed method. The proposed method is not limited to the ordering of the steps shown in FIG. 4, nor is the method envisioned to be solely limited to this example embodiment provided.

[0072] At step 410, sensitivities of the device for each of the two or more calibration solutions are determined. Said determining of sensitivities may involve calculating the ratio between an amperometer output (current) and the known concentration of creatine or creatinine of the calibration solution. In some embodiments, the concentrations of creatine or creatinine of the calibration solutions need to be determined or adjusted from an initial concentration, while in other embodiments the concentrations are provided as data accompanying the calibration solutions.

[0073] Two calibration solutions or different amounts of enzyme modulators may be provided, effectively providing two data points for determining the relationship between enzyme modulators and sensitivity. Providing more than two calibration solutions of different amounts of enzyme modulators may lead to more accurate results. One calibration solution may be chosen to have very low or no enzyme modulators, while another calibration solution may be chosen to have enzyme modulators around the same order of magnitude as the expected amount of enzyme modulators in samples. In this way, the second calibration solution a sensitivity close to the expected samples, while the first calibration solution provides sensitivities sufficiently distant from the second calibration solution to provide a good measure of the relationship between enzyme modulation and sensitivity.

[0074] At step 420, the degree of enzyme modulation is determined for each of the two or more calibration solutions. This degree of enzyme modulation is a measure of how much enzyme activity is modulated in a given solution. For example, where a bicarbonate [HCO.sub.3.sup.−] concentration and higher-than-optimum alkalinity (pH) is present, this may inhibit the enzyme activity by a certain percentage given by the degree of modulation. In such an example, the known values for [HCO.sub.3.sup.−], pH, K.sub.a and K.sub.i may be used to determine this. Alternatively, the determining may simply involve having the degree of enzyme modulation being entered as an input. In this example, the value may be known from a database or reference source, and may be input as a known, dimensionless variable for use in the method.

[0075] At step 430, the degree of enzyme modulation is determined for the sample to be measured. This determining may be similar to the determining of the degrees of enzyme modulation in step 420. The determination of the degree of enzyme modulation for the sample may also take into account the fact that rinsing may occur between samples, which may affect the contribution of pH, for example. While step 420 may be performed once for each set of calibration solutions, step 430 may be repeated for each sample being measured.

[0076] At step 440, the sensitivity of the measuring device is calculated for the sample. This step may involve determining a factor by which to adjust one of the sensitivities already determined for one of the calibration solutions. The sensitivity for the calibration solution with a degree of enzyme modulation closest to the degree of enzyme modulation of the sample may be the sensitivity that is adjusted.

[0077] The factor that adjusts the sensitivity of the calibration solution may be a function of the degree of enzyme modulation for that calibration solution and the sample. The factor may further be a function of a sensor specific constant that is an expression of the ratio between enzyme activity and the permeability of the device. This sensor specific constant may be calculated after step 420 and before step 430 and may be re-used in the calculation of sensitivities for any further samples measured using the proposed method.

[0078] At step 450, the sensitivity for the sample may be used to determine an accurate concentration of creatine or creatinine of the sample by measuring the raw output of the amperometer and dividing it by the calculated sensitivity.

[0079] It is to be understood that the present disclosure includes permutations of combinations of the optional features set out in the embodiments described above. In particular, it is to be understood that the features set out in the appended dependent claims are disclosed in combination with any other relevant independent claims that may be provided, and that this disclosure is not limited to only the combination of the features of those dependent claims with the independent claim from which they originally depend.