Mass fragmentography of creatinine proposed as a reference method

Abstract

A highly specific and accurate mass-fragmentographic method for determination of creatinine in serum and urine is described. A fixed amount of 15N2-labeled creatinine is added to a fixed amount of serum or urine. The mixture is lyophilized and creatinine is isolated by means of high-performance liquid chromatography. The creatinine is then converted into di-trifluoroacetate of the (2-hydroxy, 2-methyl)ethyl derivative and subjected to analysis with a combined gas chromatograph/mass spectrometer equipped with a multiple ion detector unit. The amount of creatinine is determined from the recordings at m/e 294 and m/e 296. These two ions correspond to the M-69 peak in the mass spectrum of derivative of unlabeled and 15N2-labeled creatinine, respectively. The relative standard deviation of the method was 3.0%. We suggest that this method can be used as a reference method for evaluation of different routine methods for determination of creatinine. The mass-fragmentographic method was compared with two routine methods for determination of serum and urinary creatinine based on the Jaffe reaction. Because of specificity, considerably lower values were obtained with the mass-fragmentographic method in the case of determination of serum creatinine, whereas almost identical results were obtained in the case of determination of urinary creatinine. For serum or urine with a wide range of concentrations of creatinine, there was a high correlation (r = .99) between the results obtained with the two routine methods and the mass-fragmentographic method. In the normal range of serum concentrations, however, the correlation between the results obtained with the two routine methods and the mass-fragmentographic method was considerably lower (r = 0.95 and 0.81, respectively). From the latter finding it is obvious that more specific methods than the direct Jaffe methods are to be preferred in the diagnosis of conditions in which renal function is affected.