Compositions and methods for improved calibration accuracy of creatinine/creatine sensors and uses thereof

A method, comprising:

• measuring, using a creatine sensor, a creatine sensor current signal (ΔI2) of a first calibration solution (CS2), where a creatine sensor sensitivity (Slope) for the creatine sensor being is based on the creatine sensor current signal (ΔI2), where the first calibration solution (CS2) has a known concentration of creatine (CR_CS2), a known concentration of creatinine (CREA_CS2), and a stable ratio of creatine to creatinine over a range of temperatures for a predefined shelf-life of the first calibration solution (CS2), and where the creatine sensor comprises a diffusion membrane over an enzyme layer and is configured to generate the creatine sensor current signal (ΔI2) based on creatine entering the enzyme through the diffusion membrane;
• measuring, using a creatinine sensor, a creatinine sensor current signal (ΔI2′) of the first calibration solution (CS2) and a creatinine sensor current signal (ΔI3′) of a second calibration solution (CS3), where the second calibration solution (CS3) has an initial known creatine concentration (CR_CS3), an initial known creatinine concentration (CREA_CS3), and an unstable ratio of creatine to creatinine that changes over the predefined shelf-life, wherein and where the creatinine sensor comprises a diffusion membrane over an enzyme layer and is configured to generate the creatinine sensor current signal (ΔI2′) based on creatinine entering the enzyme through the diffusion membrane;
• determining a first creatinine sensor sensitivity (Slope1) based on at least some of measurements obtained by the measuring performed using the creatinine sensor;
• determining a second creatinine sensor sensitivity (Slope2) based on at least some of the measurements obtained by the measuring performed using the creatinine sensor;
• measuring, using the creatine sensor, a creatine concentration of a first correction solution (COR1) having a known creatine concentration (CR_COR1), a known creatinine concentration (CREA_COR1), and a stable ratio of creatine to creatinine;
• measuring, using the creatinine sensor, a creatinine concentration of the first correction solution (COR1);
• comparing, using equal molar conversion, the creatine concentration measured by the creatine sensor to the known creatine concentration (CR_COR1) of the first correction solution and comparing, using equal molar conversion, the creatinine concentration measured by the creatinine sensor to the known creatinine concentration (CREA_COR1) of the first correction solution to determine, respectively, a creatine concentration correction factor and a creatinine concentration correction factor;
• adjusting Slope1 and Slope2 for the creatinine sensor based on the creatine concentration correction factor;
• and
• estimating a creatinine concentration in a sample based on values of the Slope, the Slope1 as adjusted, the Slope2 as adjusted, the creatine concentration correction factor, the creatinine concentration correction factor, the creatine sensor current signal (ΔI2), and the creatinine sensor current signal (ΔI2′).

A system comprising:

• a creatine sensor;
• a creatinine sensor;
• a first calibration solution (CS2) having a known concentration of creatine (CR_CS2), a known concentration of creatinine (CREA_CS2), and a stable ratio of creatine to creatinine over a range of temperatures for a predefined shelf-life of the first calibration solution (CS2);
• a second calibration solution (CS3) having an initial known creatine concentration (CR_CS3), an initial known creatinine concentration (CREA_CS3), and an unstable ratio of creatine to creatinine that changes over the predefined shelf-life;
• one or more processing devices in communication with the creatine sensor and the creatinine sensor, the one or more processing devices being configured to execute instructions;
• and
• memory storing the instructions that are executable by the one or more processing devices to perform operations comprising:
• obtaining, from the creatine sensor, a creatine sensor current signal (ΔI2) of the first calibration solution (CS2), where a creatine sensor sensitivity (Slope) for the creatine sensor is based on the creatine sensor current signal (ΔI2), and where the creatine sensor comprises a diffusion membrane over an enzyme layer and is configured to generate the creatine sensor current signal (ΔI2) based on creatine entering the enzyme through the diffusion membrane;
• obtaining, from the creatine sensor, a creatine concentration (MCR_CS3) of the second calibration solution (CS3);
• comparing, using equal molar conversion, the MCR_CS3 of the CS3 to the CR_CS3 to obtain a creatine concentration correction factor that corresponds to a difference between the MCR_CS3 and CR_CS3 caused by decay of the creatine concentration over time;
• obtaining, from the creatinine sensor:
• a creatinine sensor current signal (ΔI2′) of CS2, a creatinine sensor current signal (ΔI3′) of CS3, where the creatinine sensor comprises a diffusion membrane over an enzyme layer and is configured to generate the creatinine sensor current signal (ΔI2′) based on creatinine entering the enzyme through the diffusion membrane;
• obtaining a creatinine concentration of CS3 (MCREA_CS3);
• determining a first creatinine sensor sensitivity (Slope1) based on at least some measurements obtained from the creatinine sensor;
• determining second creatinine sensor sensitivity (Slope2) based on at least some of the measurements obtained from the creatinine sensor;
• adjusting Slope1 and Slope2 for the creatinine sensor based on the creatine concentration correction factor;
• estimating a creatinine concentration in a sample based on a sensor current signal of the sample from the creatinine sensor, the Slope1 as adjusted, and the Slope2 as adjusted;
• and
• estimating a creatine concentration in the sample based on a sensor current signal of the sample from the creatine sensor, the Slope, and the creatine concentration correction factor.