A simple radioimmunoassay for plasma cortisol: comparison with the fluorimetric method of determination

A quick and simple method for the radioimmunoassay of plasma cortisol is described. The mean morning plasma cortisol concentration in 43 normal subjects was 9.8 +/- 3.1 (S.D.) microgram/100 ml with a range of 5.0-19.5 microgram/100 ml. Mean midnight concentration in 24 normal subjects was 4.3 +/- 2.3 (S.D.) microgram/100 ml with a range of 1.4-9.6 microgram/100 ml. When compared with the fluorimetric method the mean results by radioimmunoassay of 154 routine specimens were 23% lower. In samples from unstimulated patients, regression analysis of results obtained by the two methods gave a correlation coefficient (r) of 0.93, regression line slope of 1.1, and intercept of 1.4 microgram. Mean radioimmunoassay results were 15% lower. When plasma cortisol concentration was above the normal range (greater than 30 microgram/100 ml) the regression line slope was 0.87, the intercept 17.9 microgram, r = 0.87 and mean radio immunoassay results were 37% lower. Plasma cortisol concentration in patients after insulin or Synacthen stimulation exhibited similar responses when measured by either method. Plasma cortisol concentration in normal subjects given metyrapone was lower when measured by radioimmunoassay (mean +/- S.D. = 8.7 +/- 2.7 microgram/100 ml) than when measured by fluorimetry (18.5 +/- 10.8 microgram/100 ml). The diagnostic usefulness of the two methods, ease of assay, and costs are compared.