Measurement of urea in human serum by isotope dilution mass spectrometry: A reference procedure

Background: A reference measurement procedure is needed to demonstrate the traceability of results of urea measurements in human serum. We developed a measurement procedure using the principle of isotope dilution gas chromato graphy/mass spectrometry. Methods: [C-13,N-15(2)]Urea as internal standard was added to a serum sampl e and equilibrated with endogenous nonlabeled urea. For the preparation of calibrators, the same amount of labeled urea was mixed with known amounts o f nonlabeled urea. The serum samples were treated with ethanol to remove pr oteins by precipitation. The labeled and nonlabeled urea of the samples was converted into a trimethylsilyl derivative of 2-hydroxypyrimidine. The gas chromatography/mass spectrometry system was adjusted to monitor m/z 153 an d 168 for the nonlabeled urea derivative and m/z 156 and 171 for the isotop ically labeled analogs. The results of the determination were calculated fr om peak ratios by a hyperbolic calculation function based on the theory of isotope dilution analysis. Results: The procedure was applied to control samples and patient samples a nd evaluated with respect to its trueness and precision. The standard uncer tainty of the results was 0.47-1.72%. Conclusions: This reference measurement procedure allows values to be assig ned to controls and calibrators that are traceable to the primary urea refe rence material of NIST and, therefore, to the Systeme International unit "m ole" with a low degree of uncertainty. This procedure provides a tool for t he highly accurate determination of urea in control materials as well as in patient sera. (C) 1999 American Association for Clinical Chemistry.