The effect of bilirubin and ketones on reaction rate methods for the measurement of creatinine

Abstract

While evaluating reaction rate methods for the measurement of creatinine we observed problems in analyzing icteric samples. To evaluate the significance of this observation we have used the ENI GEMSAEC to investigate the effect of bilirubin on two reaction rate procedures. In addition, we have studied the effects on the same two procedures of a number of other compounds known to interfere in the Jaffe reaction. The two procedures differ only in that procedure A measures creatinine directly on 20 μl of serum while procedure B requires 50 μl of serum due to the incorporation of a deproteinization step. At creatinine concentrations of 1.0–1.3 mg/dl the between-day C.V. for procedure A is 3.0% and for prcoedure B is 7.6%. Comparison of results for procedure A and B with results from the Technicon procedure N11b gave correlation coefficients of 0.973 and 0.992. Recovery of creatinine added to serum varied from 92% (procedure A) to 90% (procedure B) to 95% (AutoAnalyzer). The precision of procedure A, but not procedure B, is significantly decreased by increasing amounts of conjugated and unconjugated bilirubin. In addition, in specimens containing a significant amount of unconjugated bilirubin, procedure A gave lower values for creatinine than did procedure B or the AutoAnalyzer methodology. Acetoacetate, oxaloacetate, α-ketoglutarate and pyruvate all cause significant false elevations of creatinine when analyzed by procedures A and B, although the false elevations were not as great as those seen with the AutoAnalyzer method.