A simple and specific competitive protein binding assay for 24,25-dihydroxyvitamin D in human serum.

A sensitive and specific competitive protein binding assay for 24,25-dihydroxyvitamin D was developed using rat kidney cytosol as the binding protein. It also included a chromatographic step on a Sephadex LH 20 column to separate the metabolite specifically. The average value for 24, 25-dihydroxyvitamin D in serum in 19 male and female healthy subjects aged from 20 to 40 was found to be 8.24 +/- 0.34 ng/ml (1.98 +/- 0.08 . 10(-8) mol/l) mean +/- S.D. In hepatic insufficiencies the levels are higher (14.15 +/- 3.59 ng/ml) than in normal subjects. A comparison between 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D levels was made.