Comparison of three commercial calibrators for alpha-tocopherol using liquid chromatography-tandem mass spectrometry.

Abstract Objectives Alpha-tocopherol is the predominant form of vitamin E in plasma and is routinely measured to assess vitamin E status. Agreement between vitamin E assays is essential to provide consistent result interpretation. Lack of agreement among calibrators is potentially a significant obstacle to method harmonization. The aim of this study was to ascertain the agreement between commercial secondary calibrators for analysis of serum/plasma alpha-tocopherol using two methods of isotope dilution liquid chromatography–tandem mass spectrometry (LC–MSMS). Design and methods Three commercial single level calibrators (Bio-Rad Laboratories, Chromsystems Diagnostics and RECIPE) were prepared in quintuplicate in conjunction with an in-house calibrator set for alpha-tocopherol. Samples were analyzed by two methods using an Agilent-6490 LC–MSMS. Results The linearity of both methods ranged from at least 4 to 70 μmol/L. The expanded within run imprecision of both LC–MSMS methods was ± 6% at 95.4% confidence interval across the assay range. The percentage observed difference for the commercial calibrators was calculated from the observed mean against the given value of the calibrator: Bio-Rad (bias + 1.4% in both methods); Chromsystems (bias + 5.4% [first method] and + 5.0% [second method]); and RECIPE (bias − 8.9% [first method] and − 9.8% [second method]). Conclusions Results demonstrate an overall discordance between the commercial calibrators that is greater than the assay uncertainty. It is observed that the greatest deviation of the three calibrators is traceable to a different standard reference material. This lack of harmonization means that results from different laboratories may not be comparable.