25-hydroxyvitamin D (25OHD) determination is of diagnostic importance for the investigation of vitamin D deficiency and much more rarely, intoxication. Despite the name, vitamin D is a pre-hormone, being endogenously synthesised provided there is adequate sunlight. Its biological function, exerted through the active form 1,25 dihydroxyvitamin D3 (1,25(OH)2D) is to maintain calcium and phosphate levels in the blood.1 In addition, it has important roles in immune regulation.2
Demand for 25OHD and 1,25(OH)2D assays has increased substantially worldwide since the introduction of commercial kit assays, but performance is relatively poor with only just over half of laboratories achieving acceptable performance.3 A recently emerging problem is that some immunoassays underestimate 25-hydroxyvitamin D2 metabolites due to differences in affinity between the antibodies or D-binding proteins employed.4
It is likely that achievement of reliable, clinically appropriate vitamin D results will require a combination of careful use of commercial systems by skilled operators, validated reference ranges, good quality control schemes and the availability of reference methods of analysis.