How Accurate Are Assays for 25-Hydroxyvitamin D? Data from the International Vitamin D External Quality Assessment Scheme

A recent report highlighted the interlaboratory variability in serum 25-hydroxyvitamin D (25OHD) results (1). Concerns have also been raised about the apparent inability of some 25OHD assays to reliably measure 25-hydroxyvitamin D2 (25OHD2) (2)(3)(4). The accurate measurement of this metabolite is essential for the monitoring of vitamin-D-deficient patients receiving ergocalciferol, which is the only supplement used in the United States (5) and widely used elsewhere. The international Vitamin D Quality Assessment Scheme (DEQAS) has been monitoring the performance of 25OHD assays since 1989 and now has >100 registered participants in 18 countries. In essence, DEQAS is an ongoing, multicenter trial of the methods used by its participants and provides a unique opportunity to assess the accuracy and specificity of 25OHD methods as well as the analytical performance of a large number of their users. The organization of DEQAS has been described elsewhere (6), and details are available on the DEQAS website (www.deqas.org). In brief, five samples of normal human sera are sent out at 3-month intervals. Participants are asked to measure the total 25OHD concentration in each and return their results within 6 weeks. After statistical analysis of results (7), participants receive a report giving an All-Laboratory Trimmed Mean (ALTM) and Standard Deviation (SD) for each sample. A small study conducted in 1997 (8) showed that the ALTM was a good surrogate for the “true” (target) value produced by gas chromatography–mass spectrometry. The accuracy of each result is defined by its percentage bias from the ALTM. Results for each sample are also grouped by method, and a method mean (MM) is produced. The overall accuracy of each method can be assessed from the percentage bias of the method mean from the ALTM: {[(MM − ALTM)/ALTM] × 100}. The …