Automated enzymatic methods for creatinine measurement with special attention to bilirubin interference.

Four enzymatic methods for creatinine measurement were evaluated on a DuPont Dimension automatic analyzer. Biomed Creatinine-Duo-UV (BIO) and Raichem Creatinine Reagent Enzymatic (RAI) start creatinine breakdown with creatinine iminohydrolase (EC 3.5.4.21) resulting in the formation of NH4+. The Boehringer Mannheim Creatinine PAP (BM1) and SopaChem Creatinine (SOP) start the breakdown of creatinine with creatininase (EC 3.5.2.10) which yields creatine. In order to reduce bilirubin interference, the BM1 method was modified to contain K4Fe(CN)6. This substance was added with reagent 1 (BM2) or with reagent 2 (BM3). All the enzymatic creatinine methods tested displayed good linearity for concentrations up to at least 1000 mumol/l. The BIO, BM3, RAI and SOP methods showed good stability of test outcome for the tested period of a week. The outcome of the BM1 and BM2 method increased continually with time. Only the results of the RAI method were diminished by the presence of lipids. The BM1, BM2, BM3 and SOP methods showed no interference with haemoglobin, whereas this increased the outcome of the BIO method and slightly decreased the results of the RAI method. Using spiked human albumin solutions it was found that the BIO, BM2, BM3 and RAI methods displayed good resistance to interference by bilirubin or ditauro-bilirubin. The outcome of the BM1 and SOP method was strongly decreased by both bilirubin and ditauro-bilirubin. When creatinine was measured in a panel of sera originating from 100 patients with bilirubin concentrations higher than 50 mumol/l, the obtained results were in close agreement with those found for the spiked human albumin solutions.(ABSTRACT TRUNCATED AT 250 WORDS)