An automated kinetic method to determine creatinine in plasma with Jaffé reaction (alkaline picrate reagent) using a Gemsaec analyser is studied in order to argue the precision and the accuracy. The analysis of the reaction rate of a primar standard solution and of different plasmas suggests that a rapid measure is requisite in order to avoid interferences from chromogens contained in plasma which develop a same but often slower and durabler coloration at the used wavelength. The method is compared to the manual procedure using adsorption on Lloyd's reagent.