Value of low dose dexamethasone suppression test for the diagnosis of Cushing’s syndrome

Objective To assess the value of low dose dexamethasone suppression test（LDDST） for the diagnosis of Cushing’s syndrome and study the best diagnostic indications and cut-off value of LDDST. Methods Clinical data about 285 patients with Cushing’s syndrome and 146 patients without Cushing’s syndrome admitted to our department from 1991 to 2011 were retrospectively analyzed.Absolute serum and urocortisol level and inhibition rate were used as the diagnostic indications for Cushing’s syndrome.The best diagnostic indications for LDDST were obtained by comparing area under ROC curve（AUC） and calculating the best cut-off value,sensitivity and specificity of LDDST. Results Of the 285 patients with Cushing’s syndrome,154（54%）,96（33.7%）,16（5.6%）,14（4.9%）,3（1.05%）,1（0.35%）,and 1（0.35%） were diagnosed with Cushing’s disease,adrenal adenoma,AIMAH,ectopic ACTH syndrome,adrenal carcinoma,ectopic adrenal adenoma,and PPNAD,respectively.The 146 patients without Cushing’s syndrome were diagnosed with obesity,primary hypertension,PCOS,diabetes mellitus,insulin resistance and secondary amenorrhea,etc.Serum cortisol level was used as an indication for LDDST.The diagnostic value of AUC was the highest when the maximum AUC was 0.971.The sensitivity and specificity of LDDST were 97% and 98.9% respectively when the serum cortisol level was 146.5nmol/L（5.3μg/dl）,and 99.6% and 89% respectively when the serum cortisol level was 50nmol/L（1.8μg/dl） recommended by American Endocrinology Society Clinical Guideline（AESCG）. Conclusion The best indication for LDDST in diagnosis of Cushing’s syndrome is the serum cortisol level.The sensitivity and specificity of LDDST are the highest when the best cutoff value of serum cortisol level is 146.5nmol/L（5.3μg/dl）.The sensitivity of LDDST is higher while the specificity of LDDST is lower when the best cutoff value of serum cortisol level is 50nmol/L（1.8μg/dl）.