Evaluation of a predictive kinetic procedure for the enzymatic measurementof creatinine in serum

We have evaluated the analytical performance of a predictive kinetic proced ure for the enzymatic measurement of serum creatinine in comparison with eq uilibrium and kinetic fixed-time procedures. The procedure uses commerciall y available reagents and is based an calculating the total change in absorb ance expected if the reaction were to proceed to equilibrium by fitting the initial part of the absorbance-time curve to a three-parameter function by non-linear regression. Total imprecision for different concentrations of c reatinine ranges from 1.47 to 1.86% and is significantly better than that o btained for the fixed-time procedure. The results with the predictive proce dure are less dependent on creatinine amidohydrolase activity in the reagen t, pH of the reagent and temperature of analysis than are the results with the fixed-time procedure. All the procedures are interfered with by bilirub in and ascorbic acid to about the same extent. Recovery and linearity are q uite acceptable, and the estimated accuracy is below 2.1%. However, in comp arison with the equilibrium procedure, the predictive procedure tends to un derestimate creatinine concentrations at values below 100 mu mol/l, and the results obtained by the two procedures are not transferable. In conclusion , the predictive approach substantially improves the imprecision but not th e specificity of the enzymatic assay of serum creatinine.