A method of calibrating a creatinine/creatine measuring system having a creatine sensor and a creatinine sensor, comprising: measuring, with the creatine sensor, a creatine sensor current signal (ΔI2) of a first calibration solution having a known concentration of creatine (CR_CS2), a known concentration of creatinine (CREA_CS2), and a stable ratio of creatine to creatinine to establish a creatine sensor sensitivity (Slope) for the creatine sensor, wherein the stable ratio of creatine to creatinine of CS2 is of about 1.5 to 2 and can maintain the factory assigned creatinine and creatine concentrations throughout the shelf life for a minimum of 8 months without significant changes; measuring, with the creatine sensor, a measured creatine concentration (MCR_CS3) of a second calibration solution having an initial known creatine concentration (CR_CS3), an initial known creatinine concentration (CREA_CS3) and an unstable ratio of creatine to creatinine which will change over time in storage; comparing MCR_CS3 of CS3 to CR_CS3 to establish a creatine concentration correction value for CS3; measuring, with the creatinine sensor, a creatinine sensor current signal (ΔI2') of CS2 and a creatinine sensor current signal (ΔI3') of CS3, creatinine concentrations (CREA_CS2), measured creatinine concentration (MCREA_CS3), and creatine concentrations (CR_CS2) and (MCR_CS3); determining a first creatinine sensor sensitivity (Slope1); determining a second creatinine sensor sensitivity (Slope2); and estimating a creatinine concentration in a sample based on the current signal of the sample and the Slope1 and Slope2; wherein Slope=ΔI2/CR_CS2, and wherein Slope1=(MCR_CS3*ΔI2' - CR_CS2*ΔI3')/(CREA_CS2*MCR_CS3 - MCREA_CS3*CR_CS2), and wherein Slope2=(CREA_CS2*ΔI3' - MCREA_CS3*ΔI2')/(CREA_CS2*MCR_CS3 - MCREA_CS3*CR_CS2). measuring, with the creatine sensor, a creatine sensor current signal (ΔI2) of a first calibration solution having a known concentration of creatine (CR_CS2), a known concentration of creatinine (CREA_CS2), and a stable ratio of creatine to creatinine to establish a creatine sensor sensitivity (Slope) for the creatine sensor, wherein the stable ratio of creatine to creatinine of CS2 is of about 1.5 to 2 and can maintain the factory assigned creatinine and creatine concentrations throughout the shelf life for a minimum of 8 months without significant changes; measuring, with the creatine sensor, a measured creatine concentration (MCR_CS3) of a second calibration solution having an initial known creatine concentration (CR_CS3), an initial known creatinine concentration (CREA_CS3) and an unstable ratio of creatine to creatinine which will change over time in storage; comparing MCR_CS3 of CS3 to CR_CS3 to establish a creatine concentration correction value for CS3; measuring, with the creatinine sensor, a creatinine sensor current signal (ΔI2') of CS2 and a creatinine sensor current signal (ΔI3') of CS3, creatinine concentrations (CREA_CS2), measured creatinine concentration (MCREA_CS3), and creatine concentrations (CR_CS2) and (MCR_CS3); determining a first creatinine sensor sensitivity (Slope1); determining a second creatinine sensor sensitivity (Slope2); and estimating a creatinine concentration in a sample based on the current signal of the sample and the Slope1 and Slope2; wherein Slope=ΔI2/CR_CS2, and wherein Slope1=(MCR_CS3*ΔI2' - CR_CS2*ΔI3')/(CREA_CS2*MCR_CS3 - MCREA_CS3*CR_CS2), and wherein Slope2=(CREA_CS2*ΔI3' - MCREA_CS3*ΔI2')/(CREA_CS2*MCR_CS3 - MCREA_CS3*CR_CS2).