The paper describes a flow-through microsystem with optical detection for measurement of creatinine concentration in urine and post-dialysate fluid. The microsystem is made of a transparent polymer-poly(dimethylosiloxane) (PDMS). In order to determine creatinine, classical Jaffe reaction was carried out. The influence of reaction parameters (temperature and solutions flow rate) on its sensitivity and repeatability of recorded signal was investigated. Results of creatinine determination in postdialysate fluids and urine were compared to results obtained in hospital laboratories.