Determination of Plasma Testosterone by the Use of Competitive Protein Binding

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Abstract
A method is described for the determination of plasma testosterone using competitive protein binding. Two ml plasma volumes were used for each analysis in women and 0.2 ml in the case of men. Precision of the method, evaluated by determining the coefficient of variation of analyses of pools of plasma, was 3.8% for plasma from men and 5.3% for plasma from women. Accuracy was measured by analysis of 2 ml volumes of plasma to which known amounts of testosterone had been added. There was a linear relationship throughout the range examined from 0 to 750 μg/100 ml. Specificity of the present method was compared with alternative procedures which employed chromatography in different systems and derivative formation. There were no significant differences in the results when these alternative procedures were used. Ten steroids were identified which bind to testosterone binding protein with an affinity 5% or more of that of testosterone. The method is easier to perform than those using double isotope dilution to measure small amounts of testosterone. The mean plasma testosterone concentration in 18 normal women was 40 ± 14 (sd) m/g/100 ml. In 16 normal men the mean was 680±180 (sd) mμg/100 ml. Two of 3 children with feminizing testes had plasma testosterone concentrations below the normal adult female range, while a third child with the disease had a level within the normal adult female range. All 3 children with feminizing testes developed plasma testosterone concentrations above the normal adult female range after administration of human chorionic gonadotrophin.