Overnight Dexamethasone Suppression Test
Initial screening test for Cushing's syndrome in a patient with a low clinical suspicion of Cushing's. If you have a high index of suspicion of Cushing's, omit this test and go directly to the LDDST.
Patients on enzyme inducing drugs e.g. anti-convulsants may rapidly metabolise dexamethasone.
Oestrogens (e.g. pregnancy, HRT or COC) may induce cortisol binding protein and artefactually increase total cortisol levels.
Urine collection for 24 hr urinary free cortisol must not occur during this test.
Outpatient test with no particular patient preparation.
- The patient takes 1 mg dexamethasone p.o. at 2300h and the 0900h cortisol is measured the next morning (7 ml clotted blood, in red top Vacutainer).
- If the patient is collecting a 24hr urine sample for urinary free cortisol this should be completed before taking the dexamethasone.
If the 0900h cortisol value is less than 35 nmol/l the patient has shown suppression. Failure to suppress is seen in the autonomous secretion of cortisol found in Cushing's syndrome. With this cutoff, there will be a high false positive rate.
SENSITIVITY AND SPECIFICITY
Suppression in patients with Cushing's syndrome is rare with this test (2%). The reported cases metabolise dexamethasone slowly and so achieve higher circulating levels than expected. If there is strong clinical or biochemical evidence for Cushing's syndrome this test should be repeated or a formal low dose dexamethasone test performed.
Normal subjects rarely (2%) fail to suppress with overnight dexamethasone unless they are depressed (10-50%), obese (10%) or systemically unwell (10-20%). The formal low dose dexamethasone test is more specific.
This is a good screening test especially if combined with urinary free cortisol.