In the nonfasting patient, a basal blood sample should be taken at 09.00 h on day 0. Oral dexamethasone 0.5 mg to be taken strictly every 6-h for 48 h should be commenced immediately after the basal sample. A further blood test should be taken at 48 h (6 h after the last tablet).
Serum cortisol should be measured in all cases.
In healthy individuals, the basal 09.00 h serum cortisol ranges between 170 and 700 nmol/L (6-25 ^g/dL). Following dexamethasone the 48 hour sample should be <50 nmol/L (1.8 ^g/dL). Reasons for failure of suppression include Cushing's syndrome, noncompliance, glucocorticoid resistance, alcoholic pseudo-Cushing's, severe physical or psychiatric illness, and hepatic enzyme induction. Elevated cortisol-binding globulin as occurs in hyperestrogenemic states, such as pregnancy or in those taking the combined oral contraceptive pill, may result in elevated total cortisol, which fails to suppress.
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