A simplified assay for serum 25-hydroxycalciferol.

The measurement of serum 25-hydroxycalciferol (25-OH D) has proved to be a reliable index of the vitamin D nutritional status in man providing the renal 25-hydroxycalciferol-1 alpha-hydroxylase is functional. Since it is known that 25 OHD binds to alpha-globulins for its transport to the kidney, we have developed a competitive binding assay using an alpha-globulin enriched fraction (Cohn's fraction IV) as the ligand protein. A detection limit of 0.1 ng 25-OHD/tube was achieved and the non specific binding amounted to less than 3% of the initial binding. The intra- and inter-assay variations were 8.9 and 8.4% respectively. The reference values (nmol/1 +/- 2 s.d.) obtained from samples of 40 children were 50.5 +/- 21.5 with extremes of 34.8 and 95.2. The method thus proved to be a reliable and sensitive tool to assess serum 25-OHD levels.