Clinical Applications for Vitamin D Assays: What Is Known and What Is Wished for

Vitamin D is a “hot topic,” with the number of citations in PubMed exceeding 2400 in 2009, a 3-fold increase in 1 year. In the US, the number of requested 25-hydroxyvitamin D (25-OHD)9 assays is increasing exponentially. Not all of the published material has validity, however. A panel of experts was invited to address a series of questions pertaining to laboratory methods and clinical applications of available assays for 25-OHD and 1,25-dihydroxyvitamin D (1,25-OHD). What should we measure: 25-OHD3, 25-OHD2, both, or 1,25-OHD? Rosemary L. Schleicher: Our interest is in providing 25-OHD data for the National Health and Nutrition Examination Survey (NHANES). Separate estimates for 25-OHD2 and 25-OHD3—together with recent food- and supplement-intake data, questionnaire data about physical activity, sun-protection behavior and skin type, and demographic information related to race/ethnicity, season, latitude, and age—provide valuable information about the sources of vitamin D for those in the noninstitutionalized civilian US population. In addition, we will be separating and quantifying the C3 epimer of 25-OHD3, which may not be as biologically active as 25-OHD3. John Eisman: 25-OHD2 and 25-OHD3 should be measured in most clinical situations, although in many countries vitamin supplements and food fortification are moving from vitamin D2 to D3. I am unaware of clinical needs requiring knowing 25-OHD2 and 25-OHD3 separately, or the C3 epimer. There are few clinical situations where knowing the 1,25-OHD concentrations are clinically critical or overly helpful. Roger Bouillon: We need to know the combined concentration of 25-OHD3 and 25-OHD2 because both products can be converted into the active hormone 1,25-OHD. In countries where vitamin D2 supplementation is not available, the measurement of 25-OHD3 alone would be sufficient, since there is very …