Dexamethasone suppression test-overnight

Specimen:

The test is done as an outpatient screen for Cushing's syndrome.

Method:

See Cortisol.

Protocol:

Fasting not required.

Day 1

0900 hours: 5 mL blood for cortisol (optional baseline)

2300 hours: give 1 mg dexamethasone orally.

Day 2

0900 hours: 5 mL blood for cortisol.

Reference Interval:

Day 1

0900 hours cortisol: 200-650 nmol/L

Day 2

0900 hours cortisol: < 50 nmol/L.

Application:

Initial investigation when Cushing's syndrome is suspected.

Interpretation:

Day 2 cortisol should suppress to less than 50 nmol/L.

A normal result excludes Cushing's syndrome.

Failure to suppress indicates that cortisol production is not responsive to normal feedback inhibitory mechanisms.

In addition to Cushing's syndrome, failure to suppress may be seen in endogenous depression, stress, obesity, chronic alcoholism and with certain drugs eg, Phenytoin.

Failure to take the dexamethasone and abnormalities of dexamethasone absorption or metabolism should also be considered if there is failure to suppress. These can be detected by an assay of dexamethasone.

A modified test has been used in the diagnosis of endogenous depression, but is not recommended.

Reference:

Orth DN et al. In: Wilson JD and Foster DW eds. Williams Textbook of Endocrinology. 8th ed. WB Saunders, 1992.