Endocrine Abstracts

The measurement of 25OH vitamin D has tremendously evolved since the first competitive protein-binding assay. Amongst the different techniques that are now routinely employed, ELISA still represents a common tool to quantify the level of 25OH Vitamin D in individuals. Several 25OH Vitamin D ELISA assays have been developed and commercialized in the last 3 years. They all differ by the antibody used and by the technology that is applied to release 25OH Vitamin D from its binding proteins. While chemiluminescence based assays (CLIA) have been extensively evaluated and compared in the literature, virtually no studies have been conducted on the 25OH Vitamin D ELISA assays. Six commercially available 25OH Vitamin D ELISA have been compared towards increasing concentration of Vitamin D binding protein (DBP) and C3-epi-25OH Vitamin D3 (C3-epimer). The concentration of DBP in human blood is not constant and can be influenced by a number of factors including obesity, pregnancy, the use of oral contraceptives, hormone replacement therapy, liver disease, renal disease, proteinuria and intensive care. On the other hand, C3-epimer is present in the circulation of the majority of adults and children, with levels up to 93 ng/ml reported. The DIAsource Immunoassays ELISA was the only assay not being negatively biased by the addition of increasing amounts of DBP. Mean bias of −6 to −21% were observed with the other assays. Two assays were clearly impacted by the addition of C3-epimer, with mean positive bias of 28 and 30%. The performance of a 25OH Vitamin D immunoassay is mainly influenced by the choice of the antibody and by the efficiency of the method that is used to release 25OH Vitamin D from its binding proteins. Half of the assays involved in this study were significantly impacted by the addition of endogenous interferents. This confirms that 25OH Vitamin D remains a difficult assay.