Use of a common standard improves the performance of liquid chromatography-tandem mass spectrometry methods for serum 25-hydroxyvitamin-D.

BACKGROUND: Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is becoming increasingly popular for measuring 25-hydroxyvitamin-D (25-OH-D). Results submitted to the International Quality Assessment Scheme (DEQAS) have shown poor interlaboratory agreement. We investigated whether the use of a common standard would reduce interlaboratory imprecision.
METHODS: A commercial standard and two controls were distributed with the DEQAS samples in January 2008. Participants were asked to calculate the results of samples and controls using their usual standard and the commercial standard. A method questionnaire was also distributed.
RESULTS: Use of a common standard reduced the mean interlaboratory imprecision (coefficient of variation [CV]) for total 25-OH-D from 16.4% (in-house standards) to 10.4% (common standard). For 25-OH-D(3) and 25-OH-D(2), the mean CVs were reduced from 16.7% and 21.1% to 8.5% and 12.6%, respectively. Mean values obtained for total 25-OH-D using the common standard were higher by 6.1%.
CONCLUSIONS: Use of a common standard improved agreement among laboratories using LC-MS/MS methods for 25-OH-D. This suggests that problems with assay standardization contribute to interlaboratory imprecision. This may be related to the nature of the matrix used for working standards or errors in the calibration of stock standard solutions of 25-OH-D. Some participants used a gravimetric method, others UV spectrophotometry, to establish the concentration of stock solutions. Among the latter group there was uncertainty over the molar absorption coefficient of 25-OH-D solutions. We conclude that LC-MS/MS is not yet sufficiently robust to become the reference method for 25-OH-D and that gas chromatography-mass spectrometry might be a more suitable candidate.