Dexamethasone Suppression Test

Indications

Screen/diagnose Cushing’s syndrome.

Procedure

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.

Interpretation

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]

Caveats

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
Pioglitazone	Ritonavir
	Fluoxetine
	Diltiazem
	Cimetidine

See this list of a CYP inducers and inhibitors.

Alternatives

24-hour urinary free cortisol level
Midnight salivary cortisol levels

References

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.