Determination of creatinine in undiluted blood serum enzymatic flow injection analysis with optosensing

Abstract

A specific enzymatic assay for creatinine in undiluted serum samples is described, exploiting the generation of ammonia from creatinine by immobilized creatinine iminohydrolase. The ammonia produced is separated from the sample matrix by gas diffusion into an acceptor stream containing a pH-sensitive indicator. The creatinine content is quantified by monitoring the resulting colour change of the indicator by means of reflectance measurement via optical fibers, the hydrophobic gas-permeable membrane serving as a diffuse reflector. Two approaches are used to overcome the interference caused by endogenous ammonia. The first is based on enzymatic abatement of endogenous ammonia by immobilized glutamate dehydrogenase. In the second, preferable, approach, endogenous ammonia, itself a parameter of clinical interest, is measured separately prior to the enzymatic degradation by creatinine iminohydrolase. Each assay requires only 30 μl of sample solution, and the sampling frequency is 60 h −1. The relative standard deviation is approximately 3%.