Liquid chromatography–isotope dilution tandem mass spectrometry method for the measurement of urea in human serum and assignment of reference values to external quality assessment samples

Abstract Reference measurement procedures are essential for the accurate measurement of urea in human serum. The widely used gas chromatography-isotope dilution mass spectrometry (GC–IDMS) involves tedious derivatization processes. We developed and validated a simple liquid chromatography-isotope dilution tandem mass spectrometry (LC-IDMS/MS) method for the measurement of urea in human serum. In this method, 13 C, 15 N 2 -urea was spiked into serum samples as the internal standard, and pure urea certified reference material (CRM) was used as the calibration standard to ensure the traceability of the measurements to the International System of Units (SI). The sample cleanup procedure involved a simple protein precipitation step, followed by centrifugation and filtration. The developed LC-IDMS/MS method was found to be accurate and highly reproducible. The coefficient of variation (CV) was generally below 1%. Good accuracy was demonstrated by measuring serum CRMs from two National Metrology Institutes (NMIs) with relative deviations from the certified values being close to or below 1%, and measurement uncertainties of 2.0%–2.8%. The method was used to assign the reference values of serum urea in ten cycles of an external quality assessment (EQA) program for clinical laboratories over a period of four years. The robust mean of the reported results from clinical laboratories using automated analyzers were close to the assigned values from LC-IDMS/MS method. With good accuracy and precision, as well as simple sample cleanup procedure, the developed LC-IDMS/MS method can be used as a reference measurement procedure for the measurement of urea in human serum.