Relation Between Pre- and Post-Dexamethasone Test Cortisol Values in Cushing's Disease

The dexamethasone test has been widely used for diagnosing hypercortisolism. We assessed the relationship between the basal and suppressed cortisol values in urine and plasma during a low-dose dexamethasone test inpatients with proved Cushing’s disease. A statistically highly significant correlation was found (for urine cortisol: r = 0.66, p <0.0001; for plasma cortisol: r = 0.94, p <0.003). These findings imply that the lower the pretest cortisol values, the lower are the suppressed values. In patients with suspected Cushing’s syndrome and only slightly elevated cortisol excretion or low plasma concentration, the outcome might easily be considered normal. This point is particularly pertinent when assessing the post-treatment status.