Isotope Dilution–LC-MS/MS Reference Method Assessment of Serum Folate Assay Accuracy and Proficiency Testing Consensus Mean

BACKGROUND Current methods for measuring folates in clinical laboratories are competitive folate binding protein assays. These assays show a considerable lack of agreement that has implications for the comparability of data across studies as well as for long-term population studies. The development of isotope dilution–liquid chromatography–tandem mass spectrometry (ID-LC-MS/MS) reference methods permitted the evaluation of method accuracy and consistency over time. METHODS We measured 3 pools of human serum by ID-LC-MS/MS, calculated values for total folate, and distributed the same pools to participants in a national External Quality Assessment scheme. We used linear regression to compare the all-laboratory and method data with reference method values. The exercise was repeated after 18 months to assess the stability of the all-laboratory and method data. RESULTS The distributed serum pools had mass spectrometry values for folate species typical of those found in healthy individuals from populations not receiving dietary folic acid fortification. There was good agreement of the all-laboratory data set with the reference method (y =0.86x + 0.91 μg/L) at both time points. Linear regression demonstrated that the Abbott Architect showed the closest agreement with the reference method. The Roche Elecsys method was nonlinear and showed a calibration offset of 2.6 μg/L (4.57 nmol/L). CONCLUSIONS Calibration of serum folate assays traceable to higher-order reference methods increases method accuracy and improves consistency. The all-laboratory consensus mean proved sufficiently accurate and stable to be used as the target for monitoring laboratory performance.