The request for vitamin D metabolite measurements has exploded over the last several years due to the growing appreciation of the role of vitamin D in health maintenance. Much interest is shown in the measurement of 25(OH)D and to a lesser extent in the measurement of 1,25(OH)2D, but as the assays develop, requests for multiple metabolites in a single sample are increasing. Moreover, there is increased interest in the possibility that free vitamin D metabolite levels might be better markers of vitamin D status than total levels especially in individuals with altered DBP concentrations. Although immunoassays have been and remain the most prevalent assay in use today, LC-MS measurements are becoming more widespread. Each method has its advantages and disadvantages. However, as methods improve, especially for LC-MS, it is expected that LC-MS will become the more widely used method for most applications because it offers precision without the variability intrinsic to immunoassays with different antibodies. A major advance in reducing the variability between laboratories is the introduction of standards for many of the vitamin D metabolites provided by the NIST. Thus, we are approaching a time that the physician requesting these measurements from a certified laboratory can have confidence that the results are reliable in guiding clinical decision making.
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