Protocol:
(1) Some clinicians will measure serum cortisol levels at 8 AM on the day of testing.
(2) The dose of dexamethasone to administer is determined.
(2a) 1 mg in adults (a dose twice the daily maintenance dose)
(2b) 0.3 mg per square meter BSA in children (up to 1 mg)
(3) The dexamethasone is administered orally at 11 PM (from 11 PM to midnight).
(4) A blood sample for serum cortisol is drawn the next morning at 8 AM. Measurement of plasma ACTH and/or dexamethasone is also recommended.
Interpretation:
• A person with normal hypothalamic-pituitary-adrenal function will show a serum cortisol of <= 3 µg/dL. Some clinicians use < 5 µg/dL as the cutoff.
• Most patients with Cushing's syndrome will show a serum cortisol > 10 µg/dL.
• A serum level > 3 µg/dL (or > 5 µg/dL) may serve as the indication to perform one of the other dexamethasone suppression tests.
• Salivary cortisol levels may also be determined, with normals showing 0.6 to 1.1 ng/mL.
• Plasma ACTH levels should be < 20 pg/mL.
• Plasma dexamethasone values are 3-4 ng/mL (molecular weight 392.5 g). Measurement of dexamethasone levels confirms that a dose was taken and that adequate amounts were present to be effective. An abnormal result may indicate the need for a repeat test.
Causes for persistent elevation in the absence of Cushing's disease (failure to suppress, with serum levels > 3 µg/dL):
(1) stress
(2) infection
(3) acute or chronic illness
(4) obesity
(5) alcohol abuse
(6) severe depression
(7) oral contraceptive use
(8) pregnancy
(9) estrogen therapy (? cause for elevation in alcohol use, if infer cirrhosis)
(10) failure to take dexamethasone
(11) treatment of drugs that increase dexamethasone metabolism (diphenylhydantoin, phenobarbital, etc.)
(12) exogenous ACTH or glucocorticoid therapy
Advantages:
(1) There is minimal risk for complications.
(2) Testing can be performed as an outpatient.
Alternative tests:
(1) A 24 hour urinary free cortisol level expressed as micrograms per gram creatinine can help identify patients who require additional testing.
