A simplified competitive protein-binding assay for 25-hydroxycalciferol.

A single and reliable, competitive, protein-binding assay for plasma 25-hydroxycalciferol (25-OH-D), based on a previously described method which omits the chromatographic step, has been tested without the use of beta-lipoproteins. The accuracy is similar in the absence of beta-lipoproteins and in their presence, the recoveries being respectively 110-136% and 74-85%. The intraassay coefficient of variation is 7.23% in the absence versus 12.56% in the presence of beta-lipoproteins. The sensitivity of the described assay is 0.01 ng per assay tube and 1 mug/1 plasma. The mean plasma 25-OH-D level in eighteen normal Swiss volunteers aged 20 to 40 years is 39.5 +/- S.D. 9.3 mug/l. This level is similar to those found in Western Europe and U.S.A. subjects. Ten alcoholic patients without cirrhosis have significantly lower 25-OH-D plasma levels (13.0 +/- S.D. 8.9 mug/l) than appropriate age controls. This precise and accurate method is convenient for clinical studies since both chromatographic and beta-lipoproteins steps are avoided.