Use of the 2 mg 2-day test has greater specificity at high sensitivity compared to the 1 mg overnight test. However, it requires more patience on the part of the patient [2, 3].

We do not recommend 24 h urine cortisol measurement during 2 mg dexamethasone suppression test (DST) because measurement of serum cortisol concentration during the low dose dexamethasone test is simpler and more reliable than measurements of urinary steroids [3].

Do not use this test if the patient is on estrogens which increase cortisol binding globulin (CBG) and falsely elevate cortisol levels [4].

Drugs such as phenytoin, phenobarbital, phenobarbitone, carbamazepine, rifampicin, and alcohol induce hepatic enzymatic clearance of dexamethasone, mediated through CYP 3A4, thereby reducing the plasma dexamethasone concentrations and may be associated with a false positive result [5].

To evaluate for false-positive and negative responses, some experts have advocated simultaneous measurement of both cortisol and dexamethasone during dexamethasone suppression tests to ensure adequate plasma dexamethasone concentrations.