Objective: The biochemical and phenotypic presentation of mild hypercortisolism in Cushing's syndrome is often indistinguishable from that seen in pseudo-Cushing's states such as depression. Both dexamethasone suppression and corticotropin-releasing hormone (CRH) stimulation tests have been used individually to distinguish these conditions, but neither approach has achieved a diagnostic accuracy greater than 85%. Therefore, we sought to develop a combined dexamethasone-CRH test that would take advantage of the altered sensitivity of patients with Cushing's syndrome to both dexamethasone and CRH and would achieve greater accuracy in the diagnosis of Cushing's syndrome.
Design: Prospective cohort study.
Setting: Tertiary care research hospital.
Patients: A total of 58 adults referred for evaluation of mild hypercortisolism (urine free cortisol level < 1000 nmol/d). The diagnosis of Cushing's syndrome was confirmed at surgery in 39 patients. The diagnosis of a pseudo-Cushing's state was made in 19 patients on the basis of extended follow-up (mean, 28 months) without progression of cushingoid features.
Intervention: The low-dose dexamethasone suppression test, the CRH stimulation test, and the CRH stimulation test started 2 hours after completion of low-dose dexamethasone suppression (the dexamethasone-CRH test) were performed in all patients.
Main outcome measures: Sensitivity, specificity, and accuracy of the three procedures for diagnosis of Cushing's syndrome were calculated from plasma corticotropin, plasma cortisol, urine free cortisol, and urine 17-hydroxycorticosteroid values.
Results: The low-dose dexamethasone suppression test had 74% specificity, 69% sensitivity, and 71% diagnostic accuracy, using the standard criterion (17-hydroxycorticosteroid excretion level > 11.0 mumol/d on the second day of dexamethasone administration). With a urine free cortisol criterion for Cushing's syndrome of greater than 100 nmol/d, the low-dose dexamethasone suppression test had 100% specificity, 56% sensitivity, and 71% diagnostic accuracy. The CRH stimulation test without dexamethasone pretreatment had 100% specificity, 64% sensitivity, and 76% diagnostic accuracy. The diagnostic accuracy of the dexamethasone-CRH test for Cushing's syndrome was significantly greater than the accuracy of either the low-dose dexamethasone test or the CRH test alone (P < .01). A plasma cortisol concentration greater than 38 nmol/L measured 15 minutes after the administration of CRH correctly identified all cases of Cushing's syndrome and all cases of pseudo-Cushing's states (100% specificity, sensitivity, and diagnostic accuracy).
Conclusion: The dexamethasone-CRH test is a more accurate test to distinguish Cushing's syndrome from pseudo-Cushing's states in patients with mild hypercortisolism.