Abstract
The use of vitamin D testing has grown rapidly in the recent times as a result of increased interest in the role of vitamin D in health. Although the generally accepted measure of vitamin D status is circulating 25(OH)D concentration, there is little consensus on which assay method should be used. Commonly used assays include competitive proteinโbinding assay, RIA, enzyme immunoassay, chemiluminescence immunoassays, HPLC, and LCโMS/MS, each with its own advantages and disadvantages. However, there is significant interassay and interlaboratory variability in measurements. Our simulation of the published data showed that using a deficiency cutโpoint of 50โnmol/L, 57% of samples assessed using a chemiluminescence immunoassay were classified as deficient compared with 41% of samples assessed using LCโMS/MS; a 20% misclassification rate. Similar rates of misclassification were seen at 75โnmol/L. This has implications for clinical practice and decision limits for vitamin D supplementation, suggesting that cutโpoints should be assay specific rather than universal and that greater harmonization between laboratories is required. Newer assays using alternative biological samples to determine the circulating 25(OH)D have been proposed and advances in the genetics of vitamin D and the role of vitamin Dโbinding protein may improve future assay accuracy.