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A method for the determination of creatinine in human serum by isotope dilution-mass spectrometry is described. The analytical procedure comprises the following steps: Addition of [13C,15N2 )creatinine to the serum sample; ion exchange chromatography on the cation exchange resin AG 50W-X2; formation of the trimethylsilyl derivative; gas liquid chromatography-mass spectrometry (GC-MS); selected ion monitoring (SIM) at the m/z-values 329 and 332; calculation of the amount of creatinine in the serum sample from the isotope ratio, as measured by GC-MS. [13C,15N2]Creatinine was prepared by chemical synthesis. The substance is then used as internal standard for the measurement of creatinine in serum samples. The imprecision of the method was in the range from 0.35 to 1.05% (coefficient of variation) as determined by repetitive measurements of creatinine in 13 different control sera on different occasions. The lower limit of detection of the mass spectrometer in the selected ion monitoring mode is about 0.5 ng creatinine with a signal to noise ratio of 3:1 The accuracy of the method is achieved by the use of the isotope dilution principle in combination with GC-MS. In view of the high specificity and exact control of recovery, the procedure for the measurement of creatinine in human serum, as described here, may be considered as a definitive method in clinical chemistry.

Determination of Creatinine in Human Serum by Isotope Dilution-Mass Spectrometry. Definitive Methods in Clinical Chemistry, IV