A method of calibrating an enzymatic amperometric device for measuring the concentration of creatine and/or creatinine in a sample including one or more enzyme modulators, the device comprising an enzyme layer, the method comprising the following steps: determining sensitivities of the device for each of two or more calibration solutions, wherein said determining the sensitivities of the device for each calibration solutions comprises measuring device output for each of the calibration solutions and 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; measuring a concentration of enzyme modulator in the sample; receiving a concentration of enzyme modulator in each calibration solution; determining a degree of modulation for the sample to be measured, wherein the determination of the degree of modulation for the sample utilises the concentration of enzyme modulator of the sample; determining a degree of modulation for each calibration solution, wherein the determination of the degrees of modulation for the calibration solutions utilises one or more of the concentrations of enzyme modulator of each calibration solution; wherein said determining of each of the degrees of modulation comprises estimating the concentration of an enzyme modulator in the enzyme layer of the device; and calculating the sensitivity of the device for the sample, wherein the said calculating comprises adjusting the sensitivity of the device for each calibration solution by a factor comprising the determined degrees of modulation of the sample and the calibration solution, wherein said factor further comprises a ratio between two of said determined sensitivities of calibration solutions, wherein each calibration solution has a different amount of enzyme modulator; wherein prior to the determining a degree of modulation for the sample, the method further comprising aspirating an earlier sample in the device; wherein prior to the determining a degree of modulation for the sample, the method further comprises performing a partial rinse in the device; wherein said estimating the concentration of an enzyme modulator comprises determining a period of time elapsed since aspirating the earlier sample, and using it in the estimating. determining sensitivities of the device for each of two or more calibration solutions, wherein said determining the sensitivities of the device for each calibration solutions comprises measuring device output for each of the calibration solutions and 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; measuring a concentration of enzyme modulator in the sample; receiving a concentration of enzyme modulator in each calibration solution; determining a degree of modulation for the sample to be measured, wherein the determination of the degree of modulation for the sample utilises the concentration of enzyme modulator of the sample; determining a degree of modulation for each calibration solution, wherein the determination of the degrees of modulation for the calibration solutions utilises one or more of the concentrations of enzyme modulator of each calibration solution; wherein said determining of each of the degrees of modulation comprises estimating the concentration of an enzyme modulator in the enzyme layer of the device; and calculating the sensitivity of the device for the sample, wherein the said calculating comprises adjusting the sensitivity of the device for each calibration solution by a factor comprising the determined degrees of modulation of the sample and the calibration solution, wherein said factor further comprises a ratio between two of said determined sensitivities of calibration solutions, wherein each calibration solution has a different amount of enzyme modulator; wherein prior to the determining a degree of modulation for the sample, the method further comprising aspirating an earlier sample in the device; wherein prior to the determining a degree of modulation for the sample, the method further comprises performing a partial rinse in the device; wherein said estimating the concentration of an enzyme modulator comprises determining a period of time elapsed since aspirating the earlier sample, and using it in the estimating.