Overnight 1-mg Dexamethasone Suppression Test for Screening Cushing Syndrome and Mild Autonomous Cortisol Secretion (MACS): What Happens when Serum Dexamethasone Is Below Cutoff? How Frequent Is it?
Authors' conclusion
Inappropriately low serum Dex levels during the 1-mg DST may lead to false-positive results. This is associated with recurrent use of CYP3A4-inducing drugs and/or gastrointestinal abnormalities. When serum Dex is undetectable, the key reason is failure to take the medication or the use of GC (when cortisol is suppressed). Simultaneous measurement of serum cortisol and Dex allows for DST validation, improving its accuracy and avoiding unnecessary repetitions. Adherence to verbal/written recommendations and actual use of medication are critical for interpreting the test.
Study context
- Population
- Patients screened for CS or MACS with dexamethasone (Dex)-controlled 1-mg overnight DST, single centre
- Participants
- 1,300 tests
- Exposure
- Dex-undetectable vs Serum Dex 20-140 ng/dL
- Dex-undetectablen = 36
- Serum Dex 20-140 ng/dLn = 110
- Outcome
- Frequency of invalid 1-mg DSTs (serum Dex levels <140 ng/dL)
Methodological detailssingle-centre
- Single-centre
Main finding
As reported in the abstract
Invalid tests among all tests
146
11.2%
Undetectable serum Dex (<19.5 ng/dL) among invalid tests
36
24.7%
Observational data: associations, not effects.
Further findings
Reasons in Dex-undetectable tests
- Failed to take Dex correctly
- 17%
- On glucocorticoids
- 25%
- On anticonvulsants and moderate CYP3A4 inducers
- 20%
Reasons in tests with serum Dex 20-140 ng/dL
- Did not take Dex or were using GC
- 6.5%
- On anticonvulsants or CYP3A4 inducers
- 22%
- Previous GI tract abnormalities impairing drug absorption
- 13%
Original publication
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