For the low dose, overnight method, 1 mg of dexamethasone is given at 11 P.M., and the blood is drawn at 8 A.M. for a cortisol measurement (see venipuncture).
In the standard,  low dose method, urine is collected at 24-hour intervals over 3 days for measurement of cortisol. On day 2, a low dose (0.5 mg) of dexamethasone is given by mouth every 6 hours for 48-hours.

For the high dose overnight method, a baseline cortisol is measured on the morning of the test  and then 8 mg of dexamethasone is given at 11 P.M.. Blood is drawn at 8 A.M. for a cortisol measurement (see venipuncture).  For the standard high dose test, urine is collected at 24-hour intervals over 3 days for measurement of cortisol. On day 2, a high dose (2 mg) of dexamethasone is given by mouth every 6 hours for 48-hours.
 

How to Prepare for the Test

The health care provider may advise you to discontinue drugs that may affect the test. Drugs that can affect test results include barbiturates, estrogens, corticosteroids, oral contraceptives, phenytoin, spironolactone, and tetracyclines.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

• infant test or procedure preparation (birth to 1 year)
• toddler test or procedure preparation (1 to 3 years)
• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation (12 to 18 years)

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the Test is Performed

This test is performed when overproduction of cortisol is suspected.  The low dose test can help differentiate healthy people from those who produce too much cortisol.  The high dose test can help determine if the abnormality is in the pituitary gland (Cushing's Disease).  

The secretion of ACTH from the pituitary gland is normally regulated by the level of cortisol in the plasma. ACTH stimulates the adrenal cortex to produce cortisol. As plasma cortisol levels increase, ACTH secretion is suppressed, as cortisol levels decrease and ACTH increases. Dexamethasone is a synthetic steroid similar to cortisol, which suppresses ACTH secretion in normal people; giving dexamethasone should reduce ACTH levels resulting in decreased cortisol levels. People with pituitary glands which produce too much ACTH will have an abnormal response to the low dose test, but a normal response to the high dose.

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