Standardizing vitamin D assays: the way forward.

For a number of years it has been widely assumed that measurement of serum 25-hydroxyvitamin D [25(OH)D] concentration is the best approach to assessing an individual's vitamin D status. However, it has also been recognized that there is substantial within-assay variation in 25(OH)D measurement and even greater between-assay variability. Such assay variation clearly confounds attempts to define what constitutes the diagnosis of hypovitaminosis D. Importantly, assay variability makes pooling of 25(OH)D results from different studies in systematic reviews for the specific purpose of determining dose-response and/or clinical cut points at best problematic. Therefore, to develop and implement evidence-based clinical guidelines, it is essential that 25(OH)D measurement be standardized in both clinical and research laboratories. In this Perspective we outline a way forward toward achieving this goal-the Vitamin D Standardization Program (VDSP).