The measurement of vitamin D: analytical aspects

In the past quarter of a century, our understanding of the metabolism and mechanism of action of vitamin D has been elucidated. During this period, many metabolites of vitamin D have been identified and a small proportion of these assayed in blood. The ability to assay these vitamin D metabolites has led to a better appreciation of the pathological role that altered vitamin D metabolism plays in the development of diseases of calcium homeostasis. However, for many physicians it is not clear which vitamin D metabolites should be quantitated and what the information gained tells us. Of the four major circulating vitamin D metabolites in blood, only two, namely 25-hydroxyvitamin D (25OHD) and 1,25-dihydroxyvitamin D [1,25(OH)(2)D], have warranted measurement. Of these, the need for assessing serum 1,25(OH)(2)D is actually quite limited and should therefore not be considered as part of the standard vitamin D testing regimen. 25OHD, on the other hand, provides us with the single best assessment of vitamin D nutritional status and should be the only vitamin D assay typically ordered for this reason. Which of the many methods that are available should a laboratory use for quantitating either of these vitamin D metabolites? Early methods required large volumes of blood, organic solvent extractions, and extensive purification of the vitamin D metabolites prior to assay. Today, these time-consuming and costly methods have given way to a range of radioimmunoassays and enzyme-linked immunosorbent assays that can accurately and conveniently provide important information concerning an individual's vitamin D status. This review will consider when vitamin D measurements should be undertaken and how best to perform such assays.