A comparison of the standard high dose dexamethasone suppression test and the overnight 8-mg dexamethasone suppression test for the differential diagnosis of adrenocorticotropin-dependent Cushing's syndrome.

To improve the overnight 8-mg dexamethasone (DEX) suppression test (DST) for differential diagnosis of Cushing's syndrome and to develop optimal 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 Cushing's syndrome were studied (34 Cushing's disease and 7 ectopic ACTH secretion). DEX (8 mg, orally) was administered at 2300 h. Blood samples for ACTH and cortisol measurements were obtained at 0800, 0830, and 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 performed, with measurement of both urinary free cortisol and urinary 17-hydroxysteroids as indices of suppression. Optimal criteria for the diagnosis of Cushing's disease were developed for both the overnight 8-mg and the 6-day tests using receiver operating characteristic curves. The results were compared with those using the previously published criteria for diagnosis of Cushing's disease by the overnight 8-mg test (> 50% suppression of plasma cortisol at 0700-0800 h). In our patients, the previously published criterion for the overnight 8-mg test yielded high sensitivity (88%), but low specificity (57%), in making the diagnosis of Cushing's disease. When the time of cortisol measurement and the diagnostic criteria for Cushing's disease were revised to achieve 100% specificity, the sensitivity of the overnight 8-mg test was 71%, which 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 diagnostic accuracy compared to that with measurement of plasma cortisol levels alone. When the revised 8-mg overnight dexamethasone suppression test was combined with the 6-day dexamethasone suppression test, sensitivity increased to 91%, with a specificity of 100%, which was significantly better than that of the overnight 8-mg test alone (P < 0.002). We conclude that the overnight 8-mg DST has low specificity for the diagnosis of Cushing's disease when performed as originally described. However, with revised sampling times and diagnostic criteria, the overnight 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 performance of either test alone.