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dexamethasone suppression test
Indications:
- diagnosis of Cushing's syndrome
Procedure:
a) dexamethasone: 1 mg at 11 PM
b) serum cortisol at 8 AM
Reference interval:
- serum cortisol < 1.8-3 ug/dL (< 5 ug/dL) [2]
Interferences:
- False positives occur with:
1) physiologic stress, psychologic stress
2) renal failure
3) estrogen therapy
4) pharmaceutical agents
a) phenytoin
b) phenobarbital
c) carbamazepine
d) rifampin
5) endogenous depression
6) incorrect timing of dexamethasone administration or of blood sampling
7) significant obesity
8) alcoholism
9) anorexia nervosa
Principle:
- Dexamethasone suppresses pituitary secretion of ACTH which, in turn, suppresses endogenous secretion of cortisol.
- cortisol assays do NOT detect dexamethasone
Predictive value of a positive test is poor (0.6%)
Predictive value of a negative test is close to 100%.
Management:
- if 8 AM serum cortisol > 5 ug/dL
- serum ACTH*
- serum DHEA-sulfate
- urine free cortisol
- 8 mg dexamethasone suppression test* if 1 mg of dexamethasone does not fully suppress serum cortisol [2]
- serum cortisol suppresses with ACTH-secreting pituitary adenoma*
- serum cortisol does not suppress with ectopic ACTH-secreting tumor
* serum ACTH before 8 mg dexamethasone suppression test or inferior petrosal sinus ACTH
* intrapetrosal sinus sampling identifying an ACTH-secreting pituitary adenoma renders an 8 mg dexamethasone suppression test unnecessary [2]
Related
cortisol in serum
dexamethasone (Decadron, Maxidex, AK-Dex, Ozurdex)
General
provocative clinical test
References
- Saunders Manual of Medical Practice, Rakel (ed), WB Saunders,
Philadelphia, 1996, pg 652
- Medical Knowledge Self Assessment Program (MKSAP) 11, 14, 17, 18, 19.
American College of Physicians, Philadelphia 1998, 2006, 2015, 2018, 2022
- Elhomsy G, Staros EB
Medscape: Dexamethasone Suppression Test
http://emedicine.medscape.com/article/2114191-overview