Abstract
The vitamins D belong to a group of antirachitic compounds of which the most important in humans is cholecalciferol (vitamin D3), formed by the action of sunlight on the skin. Ergocalciferol (vitamin D2), a structurally similar compound, is found in invertebrates, plants, and fungi and is sometimes used as a supplement in vitamin D deficiency. Both molecules are biologically inactive and are activated by two hydroxylation steps, the first in the liver to produce 25-hydroxyvitamin D3 (25-OHD3) and D2 (25-OHD2), the second in the kidneys and other tissues to produce the bioactive form 1,25-dihdroxyvitamin D3 (1,25(OH)2D3) and D2 (1,25(OH)2D2). Serum total 25-OHD (the sum of 25-OHD3 and 25-OHD2) is universally used in the laboratory assessment of vitamin D status. Increasing workloads have led many laboratories to use commercial automated nonextraction ligand-binding assays but results from these methods can be affected by the sample matrix or presence of other hydroxylated metabolites. More rigorous liquid chromatography-tandem mass spectrometry methods are increasingly used in specialist laboratories and are the basis of Reference Measurement Procedures against which many of the commercially available immunoassays are now calibrated.
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