A COMPARISON OF THE STANDARD HIGH-DOSE DEXAMETHASONE SUPPRESSION TESTAND THE OVERNIGHT 8-MG DEXAMETHASONE SUPPRESSION TEST FOR THE DIFFERENTIAL-DIAGNOSIS OF ADRENOCORTICOTROPIN-DEPENDENT CUSHINGS-SYNDROME

To improve the overnight 8-mg dexamethasone (DEX) suppression test (DS T) for differential diagnosis of Cushing's syndrome and to develop opt imal criteria for its interpretation, we increased the number of blood samples and measured the suppression of both plasma ACTH and cortisol . Forty-one patients who were subsequently proven at surgery to have C ushing's syndrome were studied (34 Cushing's disease and 7 ectopic ACT H secretion). DEX (8 mg, orally) was administered at 2300 h. Blood sam ples for ACTH and cortisol measurements were obtained at 0800, 0830, a nd 0900 h on the day before and at 0700, 0800, 0900, and 1000 h on the morning after DEX treatment. The conventional 6-day DST was also perf ormed, with measurement of both urinary free cortisol and urinary 17-h ydroxysteroids as indices of suppression. Optimal criteria for the dia gnosis of Cushing's disease were developed for both the overnight 8-mg and the 6-day tests using receiver operating characteristic curves. T he results were compared with those using the previously published cri teria for diagnosis of Cushing's disease by the overnight 8-mg test (> 50% suppression of plasma cortisol at 0700-0800 h). In our patients, t he previously published criterion for the overnight 8-mg test yielded high sensitivity (88%), but low specificity (57%), in making the diagn osis of Cushing's disease. When the time of cortisol measurement and t he diagnostic criteria for Cushing's disease were revised to achieve 1 00% specificity, the sensitivity of the overnight 8-mg test was 71%, w hich was not significantly different from that of the 6-day test (79%; P = NS). Addition of plasma ACTH levels to the test did not improve d iagnostic accuracy compared to that with measurement of plasma cortiso l levels alone. When the revised 8-mg overnight dexamethasone suppress ion test was combined with the 6-day dexamethasone suppression test, s ensitivity increased to 91%, with a specificity of 100%, which was sig nificantly better than that of the overnight 8-mg test alone (P < 0.00 2). We conclude that the overnight 8-mg DST has low specificity for th e diagnosis of Cushing's disease when performed as originally describe d. However, with revised sampling times and diagnostic criteria, the o vernight test has sensitivity and specificity similar to those of the conventional 6-day DST. The diagnostic performance of a criterion that combines the results of both tests is better than the diagnostic perf ormance of either test alone.