An electrochemical sensor in which an interaction between one or more immobilized enzymes (e.g., creatinine iminohydrolase) and creatinine occurs, developing an electrical potential or current proportional to the quantity of ammonium (NH4+) resulting from the reaction. The electrode may use one of several technologies (e.g., enzyme immobilized by adsorption) and mechanisms (e.g., diffusion of ammonium). It is used to measure creatinine in body fluids (e.g., urine).