Overnight dexamethasone suppression test

Indications for the procedure

To identify Cushing’s Syndrome.

The Overnight Dexamethasone Suppression Test is used as an initial screening test.

Preparations for the test

No specific preparation required.

Ensure the patient is not taking steroids in any form.

Drug administration

Drug(s) Given

Dexamethasone 10 micrograms per kg body weight up to a maximum of 1 mg.
Available as scored 500 microgram tablets.
Round the dose to the dearest 250 micrograms (i.e. half a tablet).

How Given

Orally in tablet form.

Timing of Administration

Give the dose of dexamethasone between 23:00 and midnight.

Procedure

Time (minutes) Action
23:00 to 00:00 Give dexamethasone orally.
09:00 next day Take blood sample.

Samples required

Time  
09:00 Cortisol

Interpretations

In normal subjects, the morning plasma cortisol concentration is suppressed to less than 50 nmol/l.

In patients with Cushing’s syndrome, such marked suppression is not observed.

Patients taking hepatic enzyme-inducing drugs (e.g. phenytoin, phenobarbitone) may have false negative results.