Nocturnal high-dose dexamethasone suppression test in the aetiological diagnosis of Cushing's syndrome.

Seventeen patients with well-proven Cushing's syndrome (13 with Cushing's disease, 3 with adrenal tumour and 1 presenting ectopic ACTH syndrome caused by bronchial carcinoid) were investigated by using a single-dose 8 mg dexamethasone nocturnal test. The results obtained were compared with those of the classical 8 mg Liddle's test, metyrapone stimulation, plasma ACTH concentration and with the final diagnosis reached through surgery, pathologic anatomy, and/or clinical and biochemical follow-up of the patients after treatment. The diagnostic efficacy or the predictive power of the test (defined as the ratio between the number of cases in which the diagnosis was correctly predicted and the total number of cases), was at least 82.4% vs 84.6% for the classical 8 mg Liddle's test. This percentage increased to 94.1% when the results of repeated tests on three patients with conflicting data were included. It is concluded that the nocturnal high-dose dexamethasone suppression test is a valuable tool in the aetiological diagnosis of Cushing's syndrome.