Determination of creatinine in clinical samples with a creatininase reactor and an ammonia probe

A commercial nylon coil with immobilized creatininase in conjunction with a potentiometric ammonia probe is used in a continuous flow apparatus for the determination of creatinine in blood plasma and urine. The analytical characteristics of the coil are evaluated and its specific enzymatic activity is calculated. The determination of creatinine in plasma is viable provided that samples are processed quickly after they have been taken and that plasma ammonia is evaluated just before the creatinine determination. Analyses of urine require prior separation of ammonia. For plasma, 40 samples/hour can be processed with confidence. The precision of the procedure depends strongly on the plasma ammonia level and is about 5% for normal conditions.