Comparison of liquid chromatography with tandem mass spectrometry and ion-exchange chromatography by post-column ninhydrin derivatization for amino acid monitoring.

OBJECTIVES: Precise quantification of amino acids (AAs) is mandatory for successful diagnosis and monitoring of patients with metabolic diseases. We compared ion-exchange chromatography (IEC) and liquid chromatography with tandem mass spectrometry (LC-MS/MS), the two methods most commonly used in clinical laboratories for the quantification of AAs in physiological samples. DESIGN & METHODS: 123 apparently healthy children were selected for the study. The plasma samples for LC-MS/MS were prepared accordingly to the aTRAQ Kit for Physiological Fluids on Sciex 3200 Qtrap, for IEC according to the protocol from Pickering laboratories on the AA analyzer Pinnacle PCX. Results were interpreted using the Pearson correlation coefficient and the percent difference Bland-Altman test. RESULTS: The Spearman correlation coefficients of the 14 AAs that we evaluated varied from 0.67 in Tau to 0.89 in Leu and Thr. The mean differences in measurements (IEC compared to LC-MS/MS) of 11 AAs complied with our acceptance criterion of <15%, the differences of Ser and Tyr were higher (19.5% and -19.0%, respectively), and the measured concentrations of Cit were much lower in LC-MS/MS than IEC (31% difference). CONCLUSION: The two methods are sufficiently comparable for most AAs and the reference values for individual AAs did not have to be refined, with the exception of citrulline. For the monitoring of patients on therapy (e.g. patients with phenylketonuria), it is still advisable to always use the same analytical method for the quantification of AAs.