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

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).

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.

The linearity of both methods ranged from at least 4 to 70μmol/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]).

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.