The Cytoreceptor Assay for 1,25-Dihydroxyvitamin D and Its Application to Clinical Studies*

We have developed a method for the measurement of 1,25-dihydroxyvitamin D [1,25-(OH)2D] which is based on the selective internalization and subsequent binding of 1,25-(OH)2D to its specific receptor inside cultured target cells. This technique provides the means of assaying 1,25-(OH)2D directly in blood extracts without prior biochemical isolation of this hormone from other metabolites of vitamin D. The displacement of 1,25-(OH)2-[3H]D3 bound to the intracellular receptor by the extracted 1,25-(OH)2D provides a measurement of its concentration in the blood samples. The method is sensitive to 2 pg/tube and can be performed with 1- to 2-ml samples. The intra- and interassay coefficients of variation are 10.2% and 15.3%, respectively. Other major metabolites of vitamin D, at their circulating levels, do not interfere with the 1,25-(OH)2D measurement. Moreover, 1,25-(OH)2D levels determined by this direct method correlate closely (r = 0.93) with the values determined by the traditional high pressure liquid ch...