Efficacy of dexamethasone suppression test during the diagnosis of primary pigmented nodular adrenocortical disease in Chinese adrenocorticotropic hormone-independent Cushing syndrome

OBJECTIVE To evaluate the cut-off value of the ratio of 24 h urinary free cortisol (24 h UFC) levels post-dexamethasone to prior-dexamethasone in dexamethasone suppression test (DST) during the diagnosis of primary pigmented nodular adrenocortical disease in Chinese adrenocorticotropic hormone-independent Cushing syndrome. DESIGN Retrospective study. PARTICIPANTS The patients diagnosed with primary pigmented nodular adrenocortical disease (PPNAD, n = 25), bilateral macronodular adrenal hyperplasia (BMAH, n = 27), and adrenocortical adenoma (ADA, n = 84) were admitted to the Peking Union Medical College Hospital from 2001 to 2016. ESTIMATIONS Serum cortisol, adrenocorticotropic hormone (ACTH), and 24 h UFC were measured before and after low-dose dexamethasone suppression test (LDDST) and high-dose dexamethasone suppression test (HDDST). RESULTS After LDDST and HDDST, 24 h UFC elevated in patients with PPNAD (paired t-test, P = 0.007 and P = 0.001), while it remained unchanged in the BMAH group (paired t-test, P = 0.471 and P = 0.414) and decreased in the ADA group (paired t-test, P = 0.002 and P = 0.004). The 24 h UFC level after LDDST was higher in PPNAD and BMAH as compared to ADA (P < 0.017), while no significant difference was observed between PPNAD and BMAH. After HDDST, 24 h UFC was higher in patients with PPNAD as compared to that of ADA and BMAH (P < 0.017). The cut-off value of 24 h UFC (Post-L-Dex)/(Pre-L-Dex) was 1.16 with 64.0% sensitivity and 77.9% specificity, and the cut-off value of 24 h UFC (Post-H-Dex)/(Pre-H-Dex) was 1.08 with 84.0% sensitivity and 75.6% specificity. CONCLUSION The ratio of post-dexamethasone to prior-dexamethasone had a unique advantage in distinguishing PPNAD from BMAH and ADA.