An improved optical sensor and method for measuring concentration of a chemical constituent where measurement interference from other chemical compounds is present in the solution is provided. More specifically, the invention relates to a system for measuring the amount of creatinine in effluent dialysate during, before, or following a kidney dialysis procedure and a method for using the same. Alternatively, the method may be used with blood and other body fluids or solutions that contact the patient. The system may use an enzyme or other chemical process to specifically remove or convert an analyte with an intrinsic optical absorbance. By measuring the absorbance before and after the chemical process the analyte can be measured with high accuracy and specificity. The method may be extended to multiple analyte measurements using cascaded chemical processes.