Norm of Dexamethasone Suppression Test
24-hour urine values should be <50% baseline values.
|Plasma cortisol||<5 μg/dL|
|Urine-free cortisol||<25 μg/24 hours|
|Urine for 17-OHCS||4 μg/24 hours|
Positive of Dexamethasone Suppression Test
High levels of serum cortisol and 17-OHCS are present after dexamethasone is administered. Occurs in adrenal hyperplasia, adrenal incidentalomas, adrenal tumors, aldosteronism (primary), bulimia nervosa, chronic fatigue syndrome, chronic renal failure, Cushing's disease, depression, oat cell cancer of the lung, and schizophrenics with suicide attempts.
Description of Dexamethasone Suppression Test
Screening test for Cushing's disease and for depression. The test can be performed after administration of a low or high dose of dexamethasone or as an overnight test with a morning blood draw. Dexamethasone is a potent synthetic glucocorticoid that is used to test the integrity of the hypothalamic-pituitary-adrenal axis. When given to normal clients, it decreases the production of cortisol and other adrenal steroids through the usual feedback systems. In clients with Cushing's disease or depression, there is no suppression of ACTH. The low test dose is for screening. If results are positive, a high-dose test is given to determine the cause of Cushing's disease. If there is suppression with the high-dose test, it indicates a pituitary origin of the excess cortisol. If there is no suppression, it indicates an adrenal or ectopic tumor.
Professional Considerations of Dexamethasone Suppression Test
Consent form NOT required.
- Obtain a 3-L plastic container.
- Tube: Green topped, red topped, red/gray topped, or gold topped.
- Baseline values for plasma cortisol, urine-free cortisol, and urine 17-OHCS should be known.
- Obtain a 5-mL blood sample for plasma cortisol level.
- Overnight test consists of administering 1 mg of dexamethasone orally at 1100 (11:00 am) followed by venipuncture for cortisol level the next day at 0800 (8:00 am).
- The low-dose test includes a baseline measurement of urine-free cortisol or 17-OHCS followed by oral dexamethasone 0.5 mg every 6 hours for 2 days followed by a 24-hour urine for free cortisol or 17-OHCS collected on day 2.
- The high-dose test includes a baseline measure of urine-free cortisol or 17-OHCS followed by oral dexamethasone 2 mg every 6 hours for 2 days followed by a 24-hour urine for urine-free cortisol or 17-OHCS collected on day 2.
- Send the blood sample to the laboratory within 30 minutes for serum separation and freezing.
Client and Family Teaching
- Oral dexamethasone will be given at a specific time the evening before the blood sampling. The blood and urine samples will be collected at specific times the next day.
- Urine: Save all the urine voided in the 24-hour period and urinate before defecating to avoid loss of urine. If any urine is accidentally discarded, notify the physician immediately because the test results will be invalid.
Factors That Affect Results
- Failure to ingest oral dexamethasone or a radioactive scan performed within the previous 24 hours will elevate the results. For depressed clients, methylene blue is added to the dexamethasone tablets, and urinary excretion of the dye is monitored to indicate that the drug was ingested.
- False-positive results occur with acute illnesses, alcoholism, anorexia nervosa, dehydration, preclinical Cushing's syndrome (PCS), severe depression, diabetes (unstable), electroconvulsive therapy after treatment day 1, fever, malnutrition, nausea, obesity, pregnancy, high stress, and temporal lobe disease. Drugs include aspirin (drug overdose), barbiturates, carbamazepine, estrogens, glutethimide, meprobamate methaqualone, methyprylon, oral contraceptives, phenytoin, reserpine, rifampin, spironolactone, stilbestrol, and tetracycline.
- False-negative results occur with Addison's disease, hypopituitarism, and in clients who metabolize dexamethasone at an abnormally slow rate. Drugs include benzodiazepines (high dose), corticosteroids, and cyproheptadine.
- Using 1 mg of dexamethasone results in lower sensitivity in Japanese and Asian people with major depressive episodes when compared to Caucasians. Low-dose 0.5 mg DST is better in Japanese and Asian clients.
- Levels in some clients with Cushing's disease may be suppressed by 50%, but these clients can be identified by the metyrapone test.
- As a screening test for depression, it is 90% specific and 45% sensitive.
- Female survivors of sexual abuse have significantly suppressed plasma cortisol in response to dexamethasone.
- See also Metyrapone test — Serum.