A solid-phase radioimmunoassay for human corticosteroid binding globulin

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Abstract
A radioimmunoassay (RIA) for human corticosteroid binding globulin (CBG) has been developed using 125I-labelled CBG and a monospecific solid-phase CBG-antiserum (CBG-Ab-cellulose). In an RIA of serum CBG concentrations, pure CBG standards (1–100 ng protein) or samples (1 : 200) were incubated (16 h at 20 °C) with 125I-labelled CBG and CBG-Ab-cellulose. After addition of 2 ml 0·9% NaCl, the tubes were centrifuged, supernatants were aspirated and the 125I-labelled CBG bound to the CBG-Ab-cellulose pellet was counted. The specificity of the RIA was confirmed by parallel displacement curves for serial dilutions of male, female and pregnancy sera, as well as pure CBG standards. The mean ± s.d. recovery (99±8%) of pure CBG (1·6–25·0 ng) added to a diluted serum sample verified the accuracy of the method, and a good correlation (r = 0·97; n = 43) existed between serum CBG cortisol binding capacity (nmol/l) measurements and CBG concentrations (mg protein/l) measured by RIA. Intra- and interassay precisions (C.V.) at low to high serum CBG concentrations were n = 12), 20·0±4·2 in girls (n = 9), 20·7±2·7 in men (n = 6), 20·5±2·9 in women (n = 6) and 47·1 ±10·5 in pregnant women (n = 5). The sensitivity of the standard curve used in the routine RIA of serum CBG was 1·0 ng CBG/assay tube, but this could be increased to 0·2 ng/assay tube by reducing the amount of CBG-Ab-cellulose used. The RIA is suitable for both clinical and research purposes, and will aid the identification of abnormal forms of CBG and facilitate studies of the regulation of CBG production in vitro. J. Endocr. (1985) 104, 259–267