A competitive protein-binding assay for 25-hydroxyvitamin D.

1. We describe a modified competive protein-binding assay for 25-hydroxyvitamin D, which does not require the prior separation of vitamin D from 25-hydroxyvitamin D. 2. Bovine albumin was used in the buffer, at a concentration of 1mg/ml, as a protein stabilizer and lipid solubilizer. Bovine albumin from different manufactures produced very different non-specific binding of 25-(3H)hydroxycholecalciferol (ranging from 3.3 per cent to 58.9 per cent). 3. The mean concentration of 25-hydroxyvitamin D in sera from normal children in Johannesburg was 76.75 +/- 24.75 mumol/ml (30.7 +/- 9.9 ng/ml).