Recovery rate in the determination of creatinine in dog serum using the Jaffe method and an enzymatic method

In routine veterinary diagnostic procedures, serum creatinine is mostly detected by the method of Jaffe. Because of non-creatinine chromogens the accuracy of the method is limited. Therefore, we analyzed the analytical quality of the determination of creatinine in dog serum with the Jaffe method by comparing it to results obtained by enzymic analysis. Material and Methods: Incrementing amounts of creatinine were added to samples of pooled canine serum. Subsequently, the total creatinine content (both exogenously and endogenously) was determined in every sample both using the Jaffe method and via enzymic analysis each as double detection. The same procedure was performed with diluted serum samples (1:10). To estimate the accuracy of the detection methods, the recovery rate was evaluated: recovery rate (%) = actual value/true value x 100. Results: At creatinine concentrations of 87-884 mu mol/l (0.98-10 mg/dl), the analytical quality of both methods was found to be equally suitable (recovery rates of 99 +/- < 2% ). At higher ranges of 884-4420 mu mol/l (10-50 mg/dl), analytical quality depended on the method. Using enzymic analysis, we found an excellent recovery rate of 98 +/- 0.7% again. In comparison, the Jaffe method yielded a diagnostically acceptable recovery rate of 97 +/- 0.4% only up to a creatinine concentration of 2652 mu mol/l (30 mg/dl). At higher levels of serum creatinine (2652-4420 mu mol/l (30-50 mg/dl)), the recovery rate as determined by Jaff method (110 +/- 3.3%) appeared to be unacceptably high. In the diluted samples (1:10), enzymic determination of the recovery rates for creatinine resulted in values that were roughly equal (97 +/- 1.9%) to the values observed in the undiluted samples. In contrast, recovery rates from a determination of serum creatinine in diluted samples via the Jaffe method proved non-useable (72-94%). Conclusions: By means of the easy and relatively precise Jaffe method (variation coefficient: <= 3.5%) serum creatinine can be detected up to a concentration of 2600 mu mol/l (30 mg/dl) with clinically useable accuracy (recovery rates = 97-99%). As a result of this, the expected endogenous serum creatinine values in small animals without and with renal dysfunction can be measured with acceptable accuracy via Jaffe method. When evaluating renal function tests, the same is true for the measurement of exogenous serum creatinine values. At serum creatinine concentrations > 2600 mu mol/l (>30 mg/dl) or when using diluted samples, the enzymic method should be used.