The mean 1300-1600 h plasma cortisol concentration as a diagnostic test for hypercortisolism.

The 24-h mean plasma cortisol concentration was compared with the mean plasma cortisol concentrations during short subperiods of the day in 88 normal subjects and 223 patients with a very wide range of mean 24-h cortisol levels. The correlation between the 1300-1600 h mean plasma cortisol concentration and the mean 24-h plasma cortisol concentration was extremely high in all groups. Mean cortisol concentrations during this short subperiod powerfully discriminate cortisol hypersecretors (patients with Cushing's syndrome, anorexia nervosa, or prostate cancer) from normal controls. Hence, it is suggested that the mean or integrated 1300-1600 h plasma cortisol concentration can be used as a reliable afternoon cortisol test for the presence of hypercortisolism.