Plasma Corticosteroids Determined by Use of Corticosteroid-Binding Globulin and Dextran-Coated Charcoal

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Abstract
Corticosteroid-binding globulin (CBG) and dextran-coated charcoal have been employed to measure the concentration of unconjugated cor-ticosteroids in plasma. One milliliter of alkalinized plasma is extracted with methylene chloride and the extract is washed with water. The extract is dried, dissolved in a standard plasma pool containing 4-C14-cortisol, and then shaken with dextran-coated charcoal to separate protein-bound and unbound cortisol. The charcoal suspension is centri-fuged. The radioactivity in the supernatant, which is extracted and counted, is inversely related to the amount of unconjugated cortico-steroids in the original plasma. The specificity of the CBG-charcoal method as a measure of cortisol in plasma depends on 2 factors: the limited binding of most steroids other than cortisol by CBG and the low concentrations of all unconjugated steroids other than cortisol in normal human plasma. The method has a high degree of precision and accuracy. The results of analyses of plasma for corticosteroids using the CBG-charcoal method agreed closely with those obtained by using a chro-matographic-fluorometric technique which specifically measures cor-tisol.