Vitamin D in plasma: quantitation by a nonequilibrium ligand binding assay.

The concentration of vitamin D was determined in human and bovine plasma samples under various physiological and nonphysiological conditions using a nonequilibrium ligand binding assay. Prior to ligand binding analysis the vitamin D in the plasma organic extracts was purified using chromatographic procedures involving Lipidex-5000 and high performance liquid chromatography. The use of a nonequilibrium assay system greatly increased the sensitivity of our assay allowing for a minimum volume of the initial plasma sample. The vitamin D levels in plasma responded to increased sun exposure as well as to the intoxication with vitamin D3. Analysis of a plasma sample from a vitamin D-deficient patient revealed that lipid interference was not a factor in this ligand binding assay.