Dexamethasone Suppression Test Interpreter
Dexamethasone Suppression Test Interpreter
Interpret the 08:00 cortisol after a 1 mg overnight dexamethasone suppression test, using both the traditional and the more sensitive modern cut-offs.
Dexamethasone Suppression Test
Pattern → interpretation08:00 cortisol 68 nmol/L after 1 mg dexamethasone at 23:00
How the test is done
Modern cut-off: 50 nmol/L (1.8 µg/dL) · Traditional cut-off: 138 nmol/L (5 µg/dL)
- 50 nmol/L
- the modern, more sensitive cut-off — more false positives, fewer missed cases
- 138 nmol/L
- the older, more specific cut-off — fewer false positives, more missed mild disease
- dexamethasone level
- can be measured alongside cortisol to identify non-adherence or unusually rapid metabolism
Worked example
08:00 cortisol 68 nmol/L after 1 mg dexamethasone at 23:00
68 is above 50 nmol/L but not above 138 nmol/L
→ equivocal — failure to suppress by the modern cut-off, below the traditional one
The two cut-offs compared
| Cut-off | Sensitivity | Trade-off |
|---|---|---|
| 50 nmol/L (1.8 µg/dL) | Higher | More false positives from pseudo-Cushing states and enzyme induction |
| 138 nmol/L (5 µg/dL) | Lower | Fewer false positives, but mild or cyclical disease can be missed |
Reading the result, and its false positives
The overnight test gives 1 mg dexamethasone at 23:00 and measures cortisol between 08:00 and 09:00 the next morning. In a normal axis, exogenous dexamethasone suppresses ACTH and therefore endogenous cortisol; failure to suppress is the abnormal finding. The modern cut-off of 50 nmol/L (1.8 µg/dL) is more sensitive than the older 138 nmol/L (5 µg/dL) threshold, at the cost of more false positives, which is why this calculator shows both bands rather than a single pass or fail.
False positives are the main practical problem with the test, and most of them come from causes that have nothing to do with cortisol excess. Anything that induces CYP3A4 clears dexamethasone faster than expected, lowering the dose that actually reaches the pituitary — phenytoin, carbamazepine and rifampicin are the common culprits. Oestrogen raises cortisol-binding globulin, so the combined oral contraceptive pill must be stopped for six weeks beforehand. Obesity, depression, alcohol use disorder, poor sleep and shift work all produce pseudo-Cushing states that fail to suppress without true cortisol excess.
Measuring a dexamethasone level alongside the cortisol identifies non-adherence or unexpectedly rapid metabolism, either of which can produce a false failure to suppress. Because of how many things can cause a false positive, a single abnormal test should never be used alone to diagnose Cushing syndrome — a second, independent test is required before proceeding to cause-finding.
Frequently asked questions
How is the dexamethasone suppression test done?
1 mg of dexamethasone is given at 23:00, and cortisol is measured between 08:00 and 09:00 the following morning. Failure of cortisol to fall below the relevant cut-off is the abnormal finding.
Why are there two different cortisol cut-offs?
50 nmol/L (1.8 µg/dL) is the modern, more sensitive cut-off, and 138 nmol/L (5 µg/dL) is the older, more specific one. The lower cut-off catches more true disease at the cost of more false positives.
What causes a false-positive dexamethasone suppression test?
Drugs that induce CYP3A4 — phenytoin, carbamazepine, rifampicin — clear dexamethasone too fast. Oestrogen from the combined pill raises cortisol-binding globulin. Obesity, depression, alcohol use disorder, poor sleep and shift work all cause pseudo-Cushing states.
Why measure a dexamethasone level alongside cortisol?
It identifies non-adherence to the tablet or unusually rapid metabolism, either of which produces a falsely abnormal result that has nothing to do with cortisol excess.
Related calculators
References
- Nieman LK et al. The diagnosis of Cushing’s syndrome: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2008;93(5):1526–40.
- Findling JW, Raff H. Cushing’s syndrome: important issues in diagnosis and management. J Clin Endocrinol Metab. 2006;91(10):3746–53.