overnight dexamethasone suppression test in the diagnosis of cushing's disease

realizing the cause of cushing's syndrome (cs) is one of the most challenging processes in clinical endocrinology. the long high dose dexamethasone suppression test (standard test) is costly and need an extended inpatient stay. in this study we want to show the clinical utility of the overnight 8 mg dexamethasone suppression test (dst) for differential diagnosis of cs in a referral center. retrospectively from 2002-2005 we selected the patients of endocrinology ward in imam hospital who were admitted with the diagnosis of cushing syndrome and had 8 mg dst (modified test) along with classic dst. in modified test a decrease in an 8 am serum cortisol level of 50% or more is thought to indicate suppression and we compared the results of modified test with standard test. this test had been done on 42 patients: 10 male (23%) and 32 female (76%). the mean age of patients was 31.39 (15-63), 32 with proven pituitary cushing's disease, 7 with primary adrnal tumors and 3 with ectopic acth syndrome. the standard test according to 50% suppression of ufc had 90.62% sensitivity, and according to 90% suppression had 43.75% sensitivity. the sensitivity of this test was 71.85% for serum cortisol suppression. the modified test (8 mg overnight dst) had 78% sensitivity. all of these tests had 100% specificity for the diagnosis of cushing's disease. the positive predictive vale (ppv) of all of these tests was 100%. the negative predictive value (npv) of modified test for the diagnosis of cushing's disease was 58.82%. in standard test the npv of serum cortisol was 52.6%, ufc 50% had 76.9% npv and ufc 90% had 35.7% npv. the results of serum cortisol suppression in modified test is better than standard test. although 50% suppression of ufc in standard test had greater sensitivity than modified test, collecting of urine is difficult, time consuming and needing hospitalization, so we advice modified test that is much simpler and more convenient instead of standard test in the first step of evaluating the cause of cushing's syndrome in referral centers.