SAT-372 Correlation of Serum Dexamethasone and Cortisol Concentrations Post Dexamethasone 1 Mg in the Overnight Dexamethasone Suppression Test in Patients with Unilateral and Bilateral Adrenal Incidentalomas

Background: The most common functional abnormality in adrenal incidentalomas (AI) is autonomous cortisol secretion. Most guidelines recommend a 1 mg overnight dexamethasone suppression test (ONDST) as a screening test for this abnormality. There is some evidence that simultaneous measurement of post dexamethasone serum dexamethasone (SD) with serum cortisol (SC) improves the accuracy of the ONDST. We have therefore been routinely measuring SD in our unit to ensure adequate concentrations when interpreting the results of this test. Aim: To measure the correlation between SD and SC concentrations and SD with the decrement in serum cortisol levels from baseline (9 am the day before the test) to the post dexamethasone level (DSC) in the ONDST. Method: We examined the results of 57 patients with unilateral or bilateral adrenal incidentalomas who underwent a 1 mg ONDST on our unit. Using linear regression analysis, we correlated SC and SD concentrations and serum SD concentration with the DSC. We also examined whether lower levels of SD were associated with adequate suppression of SC following ONDST. Results: SD levels range <0.3 to >16, median 7.656 nmol/L. SC range <22 to 780, median 39 nmol/L and baseline cortisol range 51 to 570, median 314 nmol/L. There was no significant correlation between SD and SC (P=0.059, R squared 0.09). There was also no significant correlation with DSC (P=0.082, R squared 0.07). However, we did note adequate suppression of SC following ONDST even when the SD levels were at the lowest measurable levels (<0.3 nmol/L). Conclusion: There does not appear to be a significant correlation between SD concentrations SC or DSC in an ONDST in patients with AI. However, this small retrospective analysis indicates that even at low levels of SD, adequate suppression of SC can be observed.