Understanding Your Medical Test: A Patient Guide to Testing, Preparation, and What to Expect
This test measures ACTH hormone levels in blood after taking dexamethasone medication overnight to help diagnose adrenal gland disorders.
{"Measures ACTH hormone levels in blood plasma","Involves taking a steroid pill (dexamethasone) the night before the test","Helps doctors diagnose Cushing's syndrome and adrenal gland problems","Requires overnight fasting and specific medication timing","Blood sample is typically collected in the morning"}
Key facts at a glance
- LOINC
- 50417-5
- Short code
- ACTH Dex Suppress, ACTH Pre-Dexamethasone, ACTH 2mg DST
- Synonyms
- ACTH pre 2 mg Dex Plas-mCnc
- Category
- chemistry
- Sample
- plasma
- Container
- Lavender top (EDTA) plastic tube - must be plastic, not glass
- Units
- N/A
- Ref low
- 0
- Ref high
- 5
- Critical range
- — – 200 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- ≥ 18 years
- Method
- Chemiluminescent Immunoassay (CLIA) or Electrochemiluminescence Immunoassay (ECLIA)
- CPT
- 80424
- SNOMED
- 313295005
- Specimen volume
- 1-2 mL plasma minimum
- Stability
- Separate plasma immediately. Centrifuge within 30 minutes of collection. Freeze at -20°C if not assayed within 4 hours. Stable for up to 3 months at -70°C. Protect from light. Transport on dry ice
- Hemolysis max
- 50
- Lipemia max
- 300
- Icterus max
- 40
- TAT
- 24–72 hours
- Order
- routine
What is this test, and why might my doctor have ordered it for me?
The ACTH (Adrenocorticotropic Hormone) plasma test with dexamethasone suppression measures the level of ACTH in your blood after taking a dose of dexamethasone medication. ACTH is a hormone produced by the pituitary gland that tells your adrenal glands to release cortisol. This test is part of a procedure called the overnight dexamethasone suppression test, which helps doctors evaluate how your body responds to cortisol-like medication.
How should I prepare for this test before my appointment?
Your doctor will give you specific instructions. Generally, you will take 2 mg of dexamethasone by mouth the night before the test (usually between 11 PM and midnight). You should fast (not eat or drink anything except water) from midnight until the blood draw. Inform your doctor about all medications you are taking, as certain drugs can affect test results. Do not stop taking prescribed medications unless your doctor instructs you to do so.
What happens during the test, and how long will it take?
A healthcare professional will draw a blood sample from a vein in your arm, usually between 8 AM and 9 AM the morning after taking the dexamethasone. The blood draw takes only a few minutes. You may feel slight pain or bruising at the injection site, but this is usually temporary. After the blood is collected, you can resume your normal activities and diet.
Are there any risks or side effects that I should be aware of?
This test is generally safe. Minor risks include slight bleeding, bruising, or infection at the injection site. Some people may feel dizzy or faint during or after the blood draw. The dexamethasone dose used is very small and usually does not cause side effects, but let your doctor know if you experience any unusual symptoms.
Reference range
ACTH suppression to <5 pg/mL (or <50% of baseline) following 2 mg dexamethasone overnight suggests pituitary source; lack of suppression suggests ectopic or adrenal sourceN/A
Patient takes 2 mg dexamethasone orally at 11 PM the night before test. Blood sample collected at 8 AM the following morning. Avoid stress, strenuous exercise, and alcohol for 24 hours before test. Inform laboratory of current medications including steroids, hormones, and medications affecting hypothalamic-pituitary-adrenal axis
Clinical interpretation
How abnormal results are interpreted and what conditions they suggest.
This test is primarily used to diagnose and differentiate causes of Cushing's syndrome, a condition characterized by excess cortisol levels. In healthy individuals, taking dexamethasone should suppress ACTH production, leading to low ACTH levels. If ACTH remains high after dexamethasone, it may indicate a pituitary tumor (Cushing's disease) or ectopic ACTH-producing tumor. If ACTH is very low or undetectable, it suggests adrenal gland overproduction (adrenal Cushing's). The test also helps evaluate adrenal gland function and pituitary gland activity.
Frequently asked questions
Why do I need to take dexamethasone the night before?
How long does it take to get results?
What do abnormal results mean?
Are there any medications that can affect the test?
Do I need to fast before this test?
Sources
- 1
- 2
- 3
Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.
This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.
This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.
If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.
Medically reviewed by WellAlly Editorial Team Last reviewed August 10, 2026. · Read our editorial policy