INTRODUCTION: Cushing's syndrome is defined as chronic excess free cortisol in 
circulation. According to recent studies, midnight salivary cortisol is an 
accurate and non-stress method for screening and diagnosing Cushing's syndrome. 
However, there is limited data on midnight salivary cortisol for diagnosing 
Cushing's syndrome in the Chinese population.
METHODS: Among 61 suspected Chinese patients, 48 patients were confirmed to have 
Cushing's syndrome. We evaluated the midnight salivary cortisol, midnight serum 
cortisol and 24-hour urine free cortisol excretion for diagnosis. Midnight 
salivary cortisol was collected from 21 healthy volunteers for control purposes.
RESULTS: In the patient group, mean urine free cortisol excretion and midnight 
salivary cortisol levels were 296.50 ± 47.99 µg/day and 10.18 ± 1.29 ng/mL, 
respectively. Among the control group and normal participants, mean midnight 
salivary cortisol level was 0.53 ± 0.13 ng/mL and 0.50 ± 0.12 ng/mL, 
respectively. The cut-off value for midnight salivary cortisol was 1.7 ng/mL for 
diagnosing Cushing's syndrome, with a sensitivity of 98% and specificity of 
100%. The diagnostic performance of midnight salivary cortisol (area under the 
curve [AUC] = 0.99) was superior to that of urine free cortisol (AUC = 0.89).
CONCLUSION: Our study confirmed the good diagnostic performance of midnight 
salivary cortisol for diagnosing Cushing's syndrome in a Chinese population. 
Correlation between midnight salivary cortisol and either urine free cortisol or 
midnight serum cortisol was good. Midnight salivary cortisol is a convenient and 
precise tool for diagnosing Cushing's syndrome and can be the screening test of 
choice for Chinese populations.