Interlaboratory Variation in 25-Hydroxyvitamin D2 and 25-Hydroxyvitamin D3 Is Significantly Improved If Common Calibration Material Is Used

Measurements of 25-hydroxyvitamin D2 (D2)1 and 25-hydroxyvitamin D3 (D3) are used to detect vitamin D deficiency and monitor vitamin D supplementation. Interlaboratory variation within the same method groups makes it impossible to apply standardized reference intervals or published clinical cutoff values. Laboratories therefore must develop site-specific reference intervals. Advances in the routine use of liquid chromatography–tandem mass spectrometry (LC-MS/MS) in clinical chemistry laboratories have enabled the development of methods for measuring D2 and D3 individually (1). Despite the superior performance of LC-MS/MS methods compared with HPLC and immunoassay approaches, an examination of the UK Vitamin D External Quality Assessment Scheme (DEQAS) data for the LC-MS/MS user group indicates that interlaboratory variation exists within this method group. Calibration of D2 and D3 assays has been a continuing problem because there is no certified reference material available to test method accuracy. A recent Letter to the Editor in this journal pointed out that calibration is not the only concern with vitamin D assays. Laboratories also do not have common standard operating procedures and use a wide variety of instrumentation (2). We used commercially available calibration material (Chromsystems, …