A positive dexamethasone suppression test means your cortisol levels did not drop after taking a synthetic steroid designed to shut them down. Specifically, a morning cortisol level above 1.8 mcg/dL after taking dexamethasone the night before is considered positive, or “non-suppressed.” This result raises concern for Cushing syndrome, a condition where your body produces too much cortisol, but it does not confirm the diagnosis on its own.
How the Test Works
Your body has a built-in feedback loop that controls cortisol production. When cortisol levels rise, your brain’s pituitary gland senses this and dials back the signal telling your adrenal glands to make more. Dexamethasone is a synthetic steroid that mimics cortisol and plugs into the same receptors. When you take it, your pituitary gland should read the signal as “cortisol is high enough” and stop sending production orders. The result: your natural cortisol drops significantly by the next morning.
In a person with Cushing syndrome, this feedback loop is partially or completely broken. The source of excess cortisol, whether it’s a pituitary tumor, an adrenal tumor, or a tumor elsewhere producing hormonal signals, doesn’t respond normally to the “stop making cortisol” message. So even after taking dexamethasone, cortisol stays elevated. That failure to suppress is what makes the test positive.
What the Numbers Mean
The standard cutoff is 1.8 mcg/dL (50 nmol/L). If your morning cortisol comes back below that number, the test is negative and Cushing syndrome is effectively ruled out. This threshold catches about 95% of true cases, making it a highly sensitive screening tool.
Results fall into a few ranges worth understanding:
- Below 1.8 mcg/dL: Normal suppression. Your feedback loop is working, and Cushing syndrome is very unlikely.
- 1.8 to 5.0 mcg/dL: Sometimes called “possible autonomous cortisol secretion.” This is a gray zone that needs further investigation but may not represent true Cushing syndrome.
- Above 5.0 mcg/dL: Strongly suggests autonomous cortisol secretion, meaning your body is producing cortisol independent of normal regulation.
The 1.8 cutoff is deliberately set low to avoid missing real cases. The tradeoff is that it flags some people who don’t actually have Cushing syndrome. At that threshold, the test’s specificity is only about 72%, meaning roughly 28 out of 100 healthy people could get a falsely positive result. Some labs using more precise measurement methods have found they can raise the cutoff to around 2.4 mcg/dL without missing any true cases, which drops the false positive rate considerably.
Types of Dexamethasone Suppression Tests
The most common version is the overnight test. You take a low dose of dexamethasone (typically 1 mg) at 11 p.m. and have your blood drawn the next morning at 8 a.m. It’s simple, requires no hospital stay, and serves as the first screening step.
A two-day version exists where you take smaller doses every six hours for 48 hours, with blood drawn afterward. This uses the same 1.8 mcg/dL cutoff and similar sensitivity. An intravenous version, where dexamethasone is given through an IV in a clinical setting, uses a different threshold: cortisol is considered non-suppressed if it remains above 20% of your baseline value or above 4.7 mcg/dL. This version has reported sensitivity of 100% and specificity of 96%.
High-dose versions of the test serve a different purpose entirely. Once Cushing syndrome is confirmed, a high-dose test can help pinpoint the cause. Pituitary tumors often still respond to a large enough dose and will suppress cortisol, while adrenal tumors and certain other sources of excess cortisol will not. This distinction guides what imaging and treatment come next.
What Can Cause a False Positive
A positive result doesn’t always mean Cushing syndrome. Several medications speed up how quickly your liver breaks down dexamethasone, clearing it from your system before it has time to work. Seizure medications like carbamazepine, phenobarbital, and primidone are well-known culprits. The antibiotic rifampicin has the same effect. If you’re taking any of these, your doctor needs to know before interpreting the result.
Physical and emotional stress can also drive cortisol high enough to resist suppression. Conditions like severe depression, alcohol use disorder, obesity, and poorly controlled diabetes are recognized causes of what’s called “physiologic” or “pseudo” Cushing syndrome, where cortisol runs high for reasons unrelated to a tumor. Estrogen-containing medications, including oral contraceptives, can raise the protein that carries cortisol in the blood, artificially inflating the measured level.
What Happens After a Positive Result
A single positive screening test is never enough to diagnose Cushing syndrome. Guidelines recommend confirming the result with at least one additional test before moving forward. The two most common follow-ups are a 24-hour urine collection measuring free cortisol and a late-night salivary cortisol test, which checks whether your cortisol follows its normal pattern of dropping in the evening.
If two or more tests come back abnormal, the focus shifts to finding the source. Blood tests measuring the pituitary hormone that drives cortisol production help distinguish between a pituitary problem, an adrenal problem, and a tumor elsewhere in the body producing similar hormonal signals. Imaging with MRI or CT typically follows. The high-dose dexamethasone test may be used at this stage to further narrow down the cause.
The path from a positive screening test to a confirmed diagnosis and treatment plan can take weeks to months. Cushing syndrome is rare enough, and the testing nuanced enough, that many positive screening results turn out to be false alarms. If your test came back positive, it means more investigation is warranted, not that a diagnosis is certain.