Failure of suppression of cortisol secretion by dexamethasone is useful in identifying patients with endogenous depression.The patient takes one mg of dexamethasone at 23:00 hours. The next day plasma cortisol determinations are drawn at 16:00 and 23:00 hours. The patient should be medically healthy and physiologically stable when the test is performed.
A plasma cortisol concentration in either sample of greater than 5 ug/dL is considered a positive test. Several anticonvulsant and sedative?hypnotic drugs cause false positive tests by accelerating the rate of metabolism of dexamethasone: phenytoin, barbiturates, meprobamate, glutithimide, methyprylon, methaqualone, and carbamazepine. False negative tests are associated with hypopituitarism, adrenal cortical insufficiency, steroid therapy, indomethacin, and high dose cyproheptadine treatment.
Specimen requirement is one SST tube at each time containing a minimum of 2 mL of blood.
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