https://www.uofmhealth.org/health-library/hw6479
The night before the blood test, you will take a dexamethasone pill. The next morning, the cortisol level in your blood will be measured. If your cortisol level stays high, Cushing's syndrome may be the cause.

An ACTH test is sometimes done at the same time as the cortisol test.

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Note added at 2 hrs (2018-11-28 13:54:57 GMT)
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https://www.has-sante.fr/portail/upload/docs/application/pdf...

Un freinage minute (dexaméthasone 1 mg per os à minuit et dosage du
cortisol plasmatique à 8 heures le lendemain matin) (et/ou salivaire avec
envoi dans un laboratoire spécialisé

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Note added at 2 hrs (2018-11-28 14:00:30 GMT)
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there seem to be two types of test, overnight, and a test run over two or three days. so I think it is important to mention 'overnight'

https://www.dartmouth-hitchcock.org/endo/dexamethasone_suppr...

What does a dexamethasone suppression test involve?
The test is done either as an overnight test, or over three days. Different types of the test use differing doses of dexamethasone.
In an overnight test, you will take a dose of dexamethasone by mouth at 11 p.m., and will have a blood sample drawn at 8 a.m. to measure the levels of cortisol in your blood.
In the three-day test, your doctor will have you collect your urine during the length of the test (as in the 24-hour urine collection test). Starting on the second day of the test, you will take a dose of dexamethasone by mouth every six hours for the next 48 hours.