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Subject: High-Dose Test: Standard 2-Day (8-mg) Test
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The patient collects at least one baseline 24-hour urine, at 8 am.
The patient begins taking 2 mg of dexamethasone orally every 6 hours for a total of 8 doses, usually at 8 am, 2 pm, 8 pm, and 2 am, and the urine collections are continued.
In practice, this test is often performed immediately after completing the low-dose dexamethasone suppression test (if the test is positive).
The urine collections are assayed for urinary free cortisol and creatinine. In addition, a blood specimen can be collected 6 hours after the last dose of dexamethasone for measurement of cortisol, dexamethasone, and ACTH.
This protocol leads to the following values in normal subjects:
Urinary free cortisol excretion is <5 μg/24 hours.
Serum cortisol and plasma ACTH are low and usually undetectable.
Serum dexamethasone ranges from about 8 to 20 ng/mL.
False-positive results may occur in acute and chronic illness, alcoholism, depression, and due to certain drugs (e.g., phenytoin, phenobarbital, primidone, carbamazepine, rifampicin, and spironolactone).
Atypical or false-positive responses may occur also due to alcohol, estrogens, birth control pills, pregnancy, obesity, acute illness and stress, and severe depression.
Not a good choice for patients in whom CBG levels may be abnormal.
Noncompliance (check by measuring plasma dexamethasone).
Some patients with large ACTH-producing pituitary adenomas have marked resistance to high-dose dexamethasone suppression. In long-standing cases, nodular hyperplasia of the adrenal may develop causing autonomous cortisol production and resistance to dexamethasone test.
No suppression in 80% of cases in ectopic ACTH syndrome or nodular adrenal hyperplasia.
Urine cortisol and plasma cortisol are not decreased after high or low doses of dexamethasone in adrenal adenoma or carcinoma or ectopic ACTH syndrome.
Patients with psychiatric illness may be resistant and do not reproducibly suppress.