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We performed selective venous catheterization and sampling for ACTH in six patients with ACTH-secreting pituitary adenomas (Cushing's disease) and four patients with occult ectopic ACTH-secreting neoplasms. In five patients with Cushing's disease in whom the inferior petrosal sinus could be catheterized, ACTH levels were unequivocally higher than simultaneous peripheral values: The ratio was greater than 2.0, with a range of 2.2 to 16.7. In contrast, the inferior petrosal sinus-to-peripheral ACTH ratio in three patients with ectopic ACTH secretion was less than 1.5. In the fourth patient, an arteriovenous gradient of 6.8 was shown 2 years before a bronchial carcinoid tumor was clinically apparent. Central-to-peripheral ACTH ratios at the level of the jugular bulb and jugular vein were not diagnostic. We conclude that selective venous ACTH sampling from the inferior petrosal sinus is a reliable and useful aid in the differential diagnosis of Cushing's syndrome when standard clinical and biochemical studies are inconclusive.

We have developed a high-dose dexamethasone suppression test that can be administered overnight with a single 8-mg dose and used the new procedure in the differential diagnosis of 83 patients with Cushing's syndrome. In 76 patients with surgically or pathologically proven cause--60 with Cushing's disease, 7 with the ectopic adrenocorticotrophic hormone syndrome, and 9 with adrenal tumors--suppression of plasma cortisol levels to less than 50% of baseline indicated a diagnosis of Cushing's disease. The test had a sensitivity of 92%, a specificity of 100%, and a diagnostic accuracy of 93%. These values equal or exceed those of the standard 2-day test whether based on suppression of urinary 17-hydroxycorticosteroids or plasma cortisol. We conclude that this overnight, high-dose dexamethasone suppression test is practical and reliable in the differential diagnosis of Cushing's syndrome.