The Cushing Syndrome: An Update on Diagnostic Tests

Abstract

Study Objective: Review and evaluation of diagnostic tests for the Cushing syndrome based on reports published since 1978.

Data Identification: Studies published in the English literature from 1978 through 1989 were identified using Index Medicus and cross searching of bibliographies.

Study Selection and Data Extraction: Studies of five or more patients for general analysis and smaller studies and case reports when pertinent. To develop criteria for the corticotropin-releasing hormone (CRH) stimulation test, only studies reporting individual patient data were analyzed.

Results: No new test is clearly better than existing tests in establishing a definitive diagnosis. Among tests to determine cause, the CRH stimulation test, by newly developed criteria, has a 91% sensitivity (95% CI, 85% to 95%) and a 95% specificity (CI, 82% to 99%), and the overnight highdose dexamethasone suppression test has an 89% sensitivity (CI, 80% to 94%) and a 100% specificity (CI, 84% to 100%) for the pituitary Cushing syndrome. Magnetic resonance imaging has greater sensitivity for detecting adrenocorticotropin (ACTH)-producing pituitary adenomas than computed tomography. Inferior petrosal sinus sampling can correctly identify a pituitary cause in 88% (CI, 79% to 94%) of cases.

Conclusions: Diagnosis is still best established by using 24-hour urine free cortisol measurements or low-dose dexamethasone suppression testing. The CRH stimulation test is an outpatient alternative to determine cause, and the overnight high-dose dexamethasone test may become the test of choice along with plasma ACTH measurements by radioimmunoassay in the initial evaluation of cause. Magnetic resonance imaging should be used to evaluate the pituitary Cushing syndrome, and inferior petrosal sinus sampling is most useful in problematic cases with uncertain cause.