A method of calibrating a device for measuring the concentration of creatinine in a sample including one or more enzyme modulators, the device comprising an enzyme layer, the method comprising: determining sensitivities of the device for each of one or more calibration solutions; determining a degree of modulation for the sample to be measured, determining a degree of modulation for 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.