[Comparing overnight dexamethasone suppression test, urine free cortisol, and midnight serum cortisol for the initial diagnosis of Cushing' s syndrome].

To identify retrospectively the optimal cut-off points and the values of overnight dexamethasone suppression test, urine free cortisol, and midnight serum cortisol in the initial diagnosis of Cushing's syndrome in Chinese people.The results of overnight low-dose dexamethasone suppression test (PTC-DST), urine free cortisol (UFC) and late-night plasma total cortisol (PTC-24 h) of 102 patients who had clinically confirmed Cushing's syndrome and 102 patients without Cushing's syndrome were extracted from West China Hospital. Receiver operating characteristic (ROC) curves were drawn to identify optimal cut-off points of the three assays and their values in diagnosing Cushing' s syndrome.The optimal cut-off point (the point on the ROC curve closest to 1) for PTC-DST was set at 86 nmol/L, with 98. 7% sensitivity and 97. 2% specificity (100% sensitivity and 94. 4% specificity was achieved at 50 nmol/L 100%o sensitivity and 95. 8% specificity was achieved at 60 nmol/L). The optimal cut-off point for PTC-24 h was set at 347 nmol/L, with 93. 4% sensitivity and 98. 4% specificity (98. 9% sensitivity and 85. 2% specificity was achieved at 207 nmol/L). The optimal cut-off point for UFC was set at 230. 15 icrog/24 h, with 92. 2 sensitivity and 95. 7% specificity (96. 1% sensitivity and 85. 1% specificity was achieved at 127. 55 microg/24 h). There was no significant difference between PTC-DST and PTC-24 h in the areas under the ROC curves, whereas both had a greater area under the ROC curve than that of UFC (P< 0. 05).Compared with the recommended 50 nmol/L cut-off point for PTC-DST in the USA and the European countries, the same sensitivity and a higher specificity can be achieved at a cut-off at 60 nmol/L for the Chinese people. PTC-DST and PTC-24 h have similar values in the initial diagnosis of Cushing's syndrome, but PTC-DST is more convenient to be used in outpatient environment.