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.