Screen/diagnose Cushing’s syndrome.
The test is done in the outpatient setting.
- The patient takes dexamethasone 1 mg at bedtime (around 11 pm).
- Cortisol level is measured at 8 to 9 am the morning after taking dexamethasone.
Cortisol level after dexamethasone:
- < 50 nmol/L – rules out adrenal insufficiency (Sn 100%; Sp 91.9%). [Ceccato 2015]
- 50-140 nmol/L – borderline, possible subclinical adrenal Cushing’s syndrome.
- > 140 nmol/L – abnormal test, consistent with Cushing’s syndrome (Sn 86.8%; Sp 96.7%). [Ceccato 2015]
- Drugs that increase cortisol binding globulin (CBG) may lead to a false-positive result. Examples include estrogen containing oral contraceptive pills and mitotate. Pregnancy will also lead to elevated CBG due to high estrogen levels.
- Dexamethasone metabolism is accelerated by CYP 3A4 inducers leading to false-positive results and inhibited by CYP 3A4 inhibitors leading to false-negative results. Some examples:
|Inducers, rapid dexamethasone metabolism||Inhibitors, slow dexamethasone metabolism|
|Phenobarbital, Phenytoin, Carbamazepine, Primidone||Aprepitant/fosaprepitant|
|Rifampin Rifapentine Ethosuximide||Itraconazole|
See this list of a CYP inducers and inhibitors.
- 24-hour urinary free cortisol level
- Midnight salivary cortisol levels
- Ceccato F, Barbot M, Zilio M, Frigo AC, Albiger N, Camozzi V, Antonelli G, Plebani M, Mantero F, Boscaro M, Scaroni C. Screening Tests for Cushing’s Syndrome: Urinary Free Cortisol Role Measured by LC-MS/MS. J Clin Endocrinol Metab. 2015;100(10):3856.