The measurement of vitamin D: analytical aspects

Ann Clin Biochem 2004;41:272-281
© 2004 Royal Society of Medicine

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Review Article

Joseph E Zerwekh

Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX 75390-8885, USA

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)2D], have warranted measurement. Of these, the need for assessing serum 1,25(OH)2D is actually quite limited andshould therefore not be considered as part of the standard vitaminD testing regimen. 25OHD, on the other hand, provides us withthe single best assessment of vitamin D nutritional status andshould be the only vitamin D assay typically ordered for thisreason.

Which of the many methods that are available should a laboratoryuse for quantitating either of these vitamin D metabolites?Early methods required large volumes of blood, organic solventextractions, and extensive purification of the vitamin D metabolitesprior to assay. Today, these time-consuming and costly methodshave given way to a range of radioimmunoassays and enzyme-linkedimmunosorbent assays that can accurately and conveniently provideimportant information concerning an individual’s vitamin D status.This review will consider when vitamin D measurements shouldbe undertaken and how best to perform such assays.

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