Simple radioimmunoassay of cortisol in diluted samples of human plasma. [Tritium tracer technique]

We describe a simple radioimmunoassay of plasma cortisol, which can be performed in 3 h and which requires no purification, heating, or refrigeration steps. The plasma proteins are inhibited through direct competition between them and the antiserum at room temperature, at which the antiserum's affinity exceed that of the binding proteins. Plasma, diluted with water, is incubated for 3 h at room temperature with (/sup 3/H) cortisol and the antiserum. We compared results with those by the usual extraction method. The correlation between methods on evaluating normal samples with one antiserum was ..gamma.. = 0.954 (P < 0.001), and the slope was 0.661. With three other antisera it was ..gamma.. = 0.922 (P < 0.001) and slope 0.644. Plasmas with abnormal protein concentrations (i.e., from pregnant women, and after corticotropin adminsitration), tested to examine the validity of the method for routine use, and to define the role of the protein carriers, showed ..gamma.. = 0.859 (P < 0.001) and slope 0.726 for the four antisera used. Additional samples, assayed with diluted standards plus stripped plasma, showed a correlation with the usual extraction method of ..gamma.. = 0.945 (P < 0.001) and slope 1.026.