Appropriate Limit Value for 1 mg DST in Patients With Chronic Renal Failure

Chronic renal failure (CRF) can cause false positivity of overnight 1 mg dexamethasone suppression test (1 mg DST) in whom suspicious for Cushing Syndrome due to variability in dexamethasone bioavailability, increased cortisol secretion, disruption of diurnal rhythm, imbalance in free and bound cortisol levels because of decreased protein levels in CRF patients. In this study, we aimed to determine a specific cut-off value for 1mg DST to prevent false positivity usually seen in CRF patients according to the standard cut-off value of 1.8 mcg/dl. The data of 1038 patients who applied to Marmara University endocrinology outpatient clinic between January 01, 2018, and December 31, 2019, and who were asked for 1 mg DST were retrospectively scanned from the hospital information and management system. The patients were divided into 4 groups according to their glomerular filtration rates (GFR). 1 mg DST results were evaluated according to GFR and ROC analysis was applied to determine the new limit value for the group whose GFR \< 30 ml/min / 1.73m2 In patients classified as Group IV (the patient's GFR below 30 ml/min / 1.73m2) both 1 mg DST median and Pseudocushing Syndrome incidence were significantly higher than in other groups. The new cut-off value was found to be 3.2 mcg/dl with 100% Sensitivity and 92% specificity. This is the first study determining a specific limit value for 1 mg DST in CRF patients with GFR below 30 ml/min / 1.73m2 to avoid false positivity in these patients. Further studies are needed in more patients on hemodialysis to check the clinically more accurate 1 mg DST cutoff value in this specific population.