The corticotropin-releasing-hormone test versus the high-dose dexamethasone test in the differential diagnosis of Cushing's syndrome.

The diagnostic accuracy of the corticotropin-releasing-hormone (CRH) test was compared with that of the oral high-dose dexamethasone suppression test in the differential diagnosis of Cushing's syndrome. A false-negative response to CRH was present in 9% (2 of 22) of patients with pituitary-dependent Cushing's disease and to high-dose dexamethasone in 11% (2 of 18). All 3 patients with Cushing's syndrome due to an adrenal adenoma were unresponsive to both CRH and dexamethasone. The only patient with ectopic corticotropin secretion had a false-positive response of corticotropin to dexamethasone and no response of corticotropin to CRH. Simultaneous failure of both tests to indicate the cause of Cushing's syndrome did not occur in this series, except in 1 patient with Cushing's disease and overt macronodular hyperplasia. It is concluded that the diagnostic accuracy of the CRH test in patients with Cushing's syndrome is comparable to that of the high-dose dexamethasone test and that the highest discriminatory score in the differential diagnosis of Cushing's syndrome is achieved by using both a CRH test and a high-dose dexamethasone test.