Study objective: To determine the usefulness of the continuous intravenous dexamethasone suppression test in the differential diagnosis of patients with the Cushing syndrome.
Design: Comparison of the results of this test with those of other tests in the study population and with data from the literature.
Setting: Referral-based endocrine division in a university hospital.
Patients: One hundred and twenty-one patients with the Cushing syndrome were evaluated: corticotropin (ACTH)-secreting pituitary tumors, 90 cases; adrenal adenoma, 13; adrenal carcinoma, 9; ectopic ACTH-secreting tumors, 6; corticotropin-releasing-hormone (CRH)-secreting tumors, 2; and autonomous bilateral micronodular adrenal hyperplasia, 1. The continuous intravenous dexamethasone suppression test was done in 101 patients.
Measurements and main results: Dexamethasone (1 mg/h) was administered intravenously for 7 hours. A cortisol decrease of at least 190 nmol/L was defined as a positive response. In identification of patients with ACTH-secreting pituitary adenomas, this test had a sensitivity of 100% (95% CI, 95% to 100%), a specificity of 90% (CI, 70% to 99%), and a diagnostic accuracy of 98% (CI, 93% to 100%). Only two patients had false test results. They both had CRH-secreting tumors. Cortisol secretion was also shown to be sensitive for feedback control in other tests, however.
Conclusion: The continuous intravenous dexamethasone suppression test has better diagnostic accuracy than other tests that are currently used in the differential diagnosis of the Cushing syndrome, and this test is very convenient. The only false test results were found in patients with CRH-secreting tumors.