SCINTILLATION PROXIMITY ASSAY FOR CALCITRIOL IN SERUM WITHOUT HIGH-PRESSURE LIQUID-CHROMATOGRAPHY

A rapid isolation step for 1,25-dihydroxyvitamin D3 without high press ure liquid chromatography (HPLC) and a sensitive radioimmunoassay (RIA ) have been developed. The time required for extraction and isolation with a combination of Extrelut-1-minicolumns and Sep-Pak silica cartri dges from as little as 0.5 ml serum is only 2 h. The assay can be coun ted after 8 h of incubation. It is performed in the vial that collects the eluate, thus eliminating transfer losses and errors. No separatio n of bound and free hormone is necessary before beta-counting in the s cintillation proximity assay. The detection limit of the assay is 2.7 ng/l. The intra-assay coefficients of variation are 7.3% and 5.2% for samples with calcitriol concentrations of 31 and 148 ng/l, respectivel y. The inter-assay coefficients of variation are 11.3%, 13.3% and 16.1 % for low (16 ng/1), medium (30 ng/l) and high (148 ng/1) control pool samples, respectively. Normal values for calcitriol range from 32 to 80 ng/l. Elderly subjects, patients with reduced kidney function and p regnant women were also evaluated for their calcitriol levels. This as say correlates well with a RIA employing HPLC prepurification and char coal separation of bound/free calcitriol (r = 0.94)