Some problems associated with assay of 25-hydroxycalciferol in human serum.

Methods available for the assay of 25-hydroxycalciferol in human serum are evaluated and compared to one another. Ethanol was chosen for use in the initial extraction procedure and rat-kidney cytosol as the binding protein, although good alternative methods are also available. We used silicic acid for chromatography and found this an essential step. Reproducibility was increased when, after "bound" and "free" material were separated, an aliquot of the supernate was pipetted into the counting vial instead of the entire supernatant fluid being decanted. Beta-lipo-protein added to the assay system was of no advantage; added bovine serum albumin interfered with the assay by giving rise to high blank values. With ethanol extraction, silicic acid chromatography, rat kidney cytosol and separation on dextran-coated charcoal, sera from normal subjects showed a mean 25-hydroxycalciferol concentration of 28.5 microng/liter (range, 13.1 to 43.9) during the fall season. Coefficients of variation for a control serum were 4.9% (intra-assay) and 10.9% (interassay).