Comparison of efficacy between 1 mg overnight dexamethasone suppression test and 2-day low dose dexamethasone suppression test for diagnosis of Cushing's syndrome

Objective: To assess the sensitivity and specificity of 1 mg overnight dexamethasone suppression test (ODST) and the 2-day low dose dexamethasone suppression test (LDST) for diagnosis of Cushing's syndrome, and to compare the accuracy of both tests using cutoff value at below 5 μg/dl and at below 1.8 μg/dl. Material and Method: The present study is a retrospective study, from 1971-2007, in one academic center of 77 patients with clinical suspicion of Cushing's syndrome. Kappa statistical analysis was used to determine agreement between the two tests. Sensitivity and specificity of the tests were calculated. ROC curves were created to determine the best cutoff value of the two tests. Results: ODST has very good agreement with the more troublesome LDST and has comparable efficacy. Lowering the cutoff value from 5 μg/dl to 1.8 μg/dl does not improve the accuracy of ODST but results in decreased specificity of LDST. The best cutoff value of ODST test is ≥ 5.3 μg/dl and the best cutoff value of LDST is ≥ 5 μg/dl. Conclusion: ODST is an efficient method for diagnosis of patients suspected of having Cushing's syndrome. The attempt to lowering cutoff value does not improve the efficacy of dexamethasone suppression test.