dexamethasone suppression test (DST, Prolonged/rapid DST, Cortisol suppression test, ACTH suppression test)
المؤلف:
Kathleen Deska Pagana, Timothy J. Pagana, Theresa Noel Pagana.
المصدر:
Mosbys diagnostic and laboratory test reference
الجزء والصفحة:
15th edition , p330-332
2025-12-07
122
Type of test Blood; urine (24-hour)
Normal findings
Prolonged method
Low dose: > 50% reduction of plasma cortisol
High dose: > 50% reduction of plasma cortisol
Urinary free cortisol: < 20 mcg/24 hr (< 50 nmol/24 hr)
Rapid (overnight) method
Normal: plasma cortisol levels suppressed to < 2 mcg/dL
Test explanation and related physiology
The DST is based on pituitary adrenocorticotropic hormone (ACTH) secretion being dependent on the plasma cortisol feedback mechanism. As plasma cortisol levels increase, ACTH secretion is suppressed; as cortisol levels decrease, ACTH secretion is stimulated. Dexamethasone is a synthetic steroid (simi lar to cortisol) that normally should suppress ACTH secretion. Under normal circumstances, administration of dexamethasone results in reduced stimulation to the adrenal glands and ultimately a drop of 50% or more in plasma cortisol and 17-OCHS levels. In general, blood testing is more easily obtained and is equally accurate and reliable as urine testing, especially in low- dose DST. This important feedback system does not function properly in patients with hypercortisol states.
When elevated cortisol levels caused by an ACTH-producing pituitary tumor lead to bilateral adrenal hyperplasia (Cushing disease), the pituitary gland is reset upward and responds only to high plasma levels of dexamethasone. There is no response to low-dose dexamethasone. In Cushing syndrome caused by adrenal adenoma or cancer (which acts autonomously), cortisol secretion continues despite a suppression in ACTH even at high doses of dexamethasone. Likewise, when Cushing syndrome is caused by an ectopic ACTH-producing tumor (e.g., lung cancer), that tumor is also considered autonomous and will continue to secrete ACTH despite high-dose suppression with dexamethasone. ACTH and plasma cortisol levels are measured during this test. Dexamethasone levels may also be measured to ensure adequacy of suppressive doses administered.
When hypercortisol is caused by:
• Bilateral adrenal hyperplasia (Cushing disease)
❍ Low dose: no change
❍ High dose: > 50% reduction of plasma cortisol and ACTH is elevated
• Adrenal adenoma or carcinoma (primary hypercortisolism)
❍ Low dose: no change
❍ High dose: no change
❍ ACTH is undetectable or low
• Ectopic ACTH-producing tumor ❍ Low dose: no change ❍ High dose: no change ❍ ACTH is normal to elevated
The DST may also identify depressed persons likely to respond to electroconvulsive therapy or antidepressants rather than to psychological or social interventions. ACTH production will not be fully suppressed after administration of low-dose dexamethasone in these patients.
The prolonged DST (high-dose DST—usually 8 mg of dexamethasone) can be performed over a 2-day period on an outpatient basis. The rapid DST (low dose—1 mg of dexamethasone) is easily and quickly performed and is used primarily as a screening test to diagnose Cushing syndrome. It is less accurate and informative than the prolonged DST, but when its results are normal, the diagnosis of Cushing syndrome can be safely excluded. The high dose DST is used if the results of the low-dose DST are equivocal. Corticotropin-releasing hormone can be added to dexamethasone to increase the accuracy of this test in order to differentiate Cushing syndrome from “pseudo-Cushing states.” In pseudo-Cushing, one would expect elevations of ACTH and cortisol levels.
Interfering factors
• Physical and emotional stress can elevate ACTH release.
* Drugs that can affect test results include barbiturates, estro gens, oral contraceptives, phenytoin, spironolactone, steroids, and tetracyclines.
Procedure and patient care
Before
* Explain the procedure (prolonged or rapid test) to the patient.
• Check with the lab to see if fasting is needed.
During
• There are several documented methods of performing this test by varying the dose and duration of testing.
Prolonged test
• Obtain a baseline 24-hour urine collection for urinary free cortisol. See inside front cover for Routine Urine Testing.
• Collect blood for determination of baseline plasma cortisol levels, if indicated.
• Collect 24-hour urine specimens daily over a 2-day period. Because 2 continuous days of urine collections are needed, no urine specimens are discarded except for the first voided specimen on day 1, after which the collection begins.
• On days 1 and 2, administer a low dose of dexamethasone by mouth every 6 hours for 48 hours.
• Administer the dexamethasone with milk or an antacid to pre vent gastric irritation.
• Note that the urine samples for free cortisol do not need a preservative.
• Note that creatinine is measured in all the 24-hour urine col lections to demonstrate their accuracy and adequacy.
• Keep the urine specimens refrigerated or on ice during the collection period.
Rapid test
• Give the patient a low dose of dexamethasone by mouth at bedtime.
• Administer the dexamethasone with milk or an antacid.
• Attempt to ensure a good night’s sleep. However, use sedative-hypnotic drugs only if absolutely necessary.
• At 8 am the next morning, draw fasting blood for determination of plasma cortisol level before the patient arises.
• If no cortisol suppression occurs after the dose of dexamethasone, administer a higher dose at bedtime and obtain a cortisol level as described previously. This is referred to as the overnight dexamethasone suppression test. Patients with adrenal hyperplasia will suppress. Patients with adrenal or ectopic tumors will not suppress.
After
• Assess the patient for steroid-induced side effects by monitoring glucose levels and potassium levels.
Abnormal findings
- Cushing syndrome
- Cushing disease
- Ectopic ACTH-producing tumors
- Adrenal adenoma or carcinoma
- Bilateral adrenal hyperplasia
- Mental depression
- Hyperthyroidism
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