Dexamethasone Suppression for Cushing Syndrome |
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Some medical centers recommend the 1-mg overnight dexamethasone suppression test followed by morning plasma cortisol measurement as the preferred screening test for Cushing syndrome. However, this test generally does not perform as well as urinary free cortisol. Obesity, depression, alcoholism, high estrogen states, and uremia can cause false positive results. Chronic renal failure and liver failure can cause false negative results. The 1-mg overnight dexamethasone suppression test is most useful in patients with ambiguous urinary free cortisol values.
The single-dose overnight dexamethasone suppression test is performed by administering 1 mg of dexamethasone p.o. at 23:00 and collecting a specimen for plasma cortisol measurement at 08:00 the next morning. Normally, plasma cortisol should be suppressed below 5 ug/dL. Cortisol levels < 5 ug/dL virtually exclude the diagnosis of Cushing syndrome, while levels > 10 ug/dL support the diagnosis.
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