Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (897 page)

   Serum cortisol and plasma ACTH are low and usually undetectable.

   Serum dexamethasone ranges from about 8 to 20 ng/mL.

   Limitations of All Tests

   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.

DIGOXIN
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   Definition

   Digoxin is an analogue of digitoxin.