This test requires administration of dexamethasone, an oral steroid. Dexamethasone suppresses levels of circulating adrenal steroid hormones in normal people but fails to suppress them in patients with Cushing's syndrome and some forms of clinical depression.

Purpose

To diagnose Cushing's syndrome
To aid diagnosis of clinical depression

Patient Preparation

Explain the purpose of the test.
Inform the patient that the test requires two blood samples drawn after administration of dexamethasone. Tell him who will perform the venipuncture and when and that he may experience transient discomfort from the needle puncture and the tourniquet.
Restrict food and fluids for 10 to 12 hours before the test.
Many medications, including corticosteroids, oral contraceptives, lithium, methadone, aspirin, diuretics, morphine, and monoamine oxidase (MAO) inhibitors, can affect the accuracy of test results. If possible, don't administer any of these medications for 24 to 48 hours before the test.

Procedure And Posttest Care

On the first day, give the patient 1 mg of dexamethasone at 11 p.m. On the next day, collect blood samples at 4 p.m. and 11 p.m. More frequent sampling may increase the likelihood of measuring a nonsuppressed cortisol peak.
If a hematoma develops at the venipuncture site, apply warm soaks.

Reference values

A cortisol level of 5g/dl (140 nmol/L) or greater indicates failure of dexamethasone suppression.

Abnormal Findings

A normal test result doesn't rule out major depression, but an abnormal result strengthens a clinically based diagnosis. Failure of suppression occurs in patients with Cushing's syndrome, severe stress, and depression that's likely to respond to treatment with antidepressants.

Interfering Factors

Diabetes mellitus, pregnancy, and severe stress, such as trauma, severe weight loss, dehydration, and acute alcohol withdrawal (possible false-positive)
Certain drugs, particularly barbiturates or phenytoin, within 3 weeks of the test (possible false-positive)
Caffeine consumed after midnight the night before the test (possible falsepositive)
Failure to withhold corticosteroids, oral contraceptives, lithium, methadone, aspirin, diuretics, morphine, or MAO inhibitors before the test

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