Combined enzymic-Jaffé method for determination of creatinine in serum

Concentrations of creatinine, as determined in serum by a method involving the combined use of creatinine amidohydrolase (EC 3.5.2.10) and alkaline sodium picrate were found to be factitiously low, owing to a reversal of the enzyme reaction. This effect could be eliminated by converting creatine, the product of the enzymic reaction, to creatine phosphate. The combined enzymic-Jaffé method was therefore modified to include creatine amidohydrolase, creatine kinase, ATP, and Mg2+ in the reaction mixture. The modified method has good precision. We saw no significant interferences by relatively high concentrations of acetone, acetylacetone, ADP, creatine, creatine phosphate, glucose, glycocyamine, or pyruvate. Likewise, no interferences were evident with icteric, lipemic, or hemolytic serum samples. There was an excellent agreement between creatinine values obtained with our method and by a reference method based on isotope dilution-mass fragmentography. Our method is considerably simpler than the fully enzymic method for determination of creatinine and might be a method of choice if a high accuracy is desired.