No single lab test is perfect, and usually, several are needed. The three most common tests used to diagnose Cushing's syndrome are the 24-hour urinary free cortisol test, the measurement of midnight plasma cortisol or late-night salivary cortisol, and the low-dose dexamethasone suppression test (LDDST). Another test, the dexamethasone-corticotropin-releasing hormone (dexamethasone-CRH) test, may be needed to distinguish Cushing's syndrome from other causes of excess cortisol.

24-hour urinary free cortisol test:
In this test, a person's urine is collected several times over a 24-hour period and tested for cortisol.

Midnight plasma cortisol and late-night salivary cortisol measurements:
The midnight plasma cortisol test measures cortisol concentrations in blood. Cortisol production is normally suppressed at night, but in Cushing's syndrome, this suppression doesn't occur. The test generally requires a 48-hour hospital stay to avoid falsely elevated cortisol levels due to stress.

LDDST:
In this test, a person is given a low dose of dexamethasone, a synthetic glucocorticoid, by mouth every six hours for two days. Urine is collected before dexamethasone is administered and several times on each day of the test. A modified LDDST uses a onetime overnight dose.

Cortisol and other glucocorticoids signal the pituitary to release less adrenocorticotropic hormone (ACTH). So, the normal response after taking dexamethasone is a drop in blood and urine cortisol levels. If cortisol levels do not drop, Cushing's syndrome is suspected.

Dexamethasone-CRH test
Some people have high cortisol levels, but they do not develop the progressive effects of Cushing's syndrome, such as muscle weakness, fractures, and thinning of the skin. These people may have pseudo-Cushing's syndrome. Pseudo-Cushing's syndrome does not have the same long-term effects on health as Cushing's syndrome.

The dexamethasone-CRH test rapidly distinguishes pseudo-Cushing's syndrome from mild cases of Cushing's syndrome. This test combines the LDDST and a corticotropin-releasing hormone (CRH) stimulation test. Elevations of cortisol during the dexamethasone-CRH test suggest Cushing's syndrome.

From http://www.endocrine.niddk.nih.gov/pubs/cushings/cushings.htm#diagnosis

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