We do not recommend 24 h urine collection for cortisol because measurement of serum cortisol concentration during the low dose dexamethasone test is simpler than, and as accurate and reliable as, measurements of urinary steroids [2].

The diagnostic accuracy of the dexamethasone-CRH test for Cushing’s syndrome can be significantly greater than the accuracy of either the low-dose dexamethasone test or the CRH test alone [1].

Do not use this test if the patient is on estrogens that increase cortisol-binding globulin (CBG) and falsely elevate cortisol levels [3].

Drugs such as phenytoin, 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 [4].

To evaluate for false-positive and negative responses, some experts have advocated simultaneous measurement of both cortisol and dexamethasone for these tests to ensure adequate plasma dexamethasone concentrations [5].