Evaluation of the 6-hour integrated concentration of cortisol as a diagnostic procedure for Cushing's syndrome.

The diagnostic value of the 24-h integrated concentration (IC) test of cortisol (IC-24-F) was found to be superior to the value of both urinary 17OHCS and urinary free-cortisol tests. The IC-24-F test is too cumbersome for widespread clinical use. The purpose of the present study was to evaluate the diagnostic value of an abbreviated and practical 6-h IC of cortisol (IC-6-F) test. The IC of cortisol (IC-F) was measured in 68 normal subjects and 13 patients with surgically proven Cushing's syndrome. A portable nonthrombogenic constant blood withdrawal system was used over a 24-h period. The IC-F was measured in plasma withdrawn during each 1/2-h period (IC-1/2-F). The mean of 12 consecutive measurements of IC-1/2-F yielded the IC-6-F. The mean of all the IC-1/2-F collected over a 24-h period constituted the IC-24-F. The IC-1/2-F of the patients and their IC-6-F from 0800-1400 h, 1400-2000 h, and 2000-0200 h overlapped the corresponding levels in the control subjects. There was no overlap between the IC-24-F and the IC-6-F (from 2000-0200 h) of the patients and the control subjects. It was concluded that the diagnostic value of a 6-h IC-F test conducted during the afternoon and early part of the might is equal to the diagnostic value of the 24-h IC-F test.