Understanding Your Blood Test: A Patient's Guide to What It Is, How It Works, and Why It Matters
This test measures ACTH (adrenocorticotropic hormone) levels in blood after taking low-dose dexamethasone to help diagnose Cushing's syndrome.
{"Measures ACTH hormone levels after dexamethasone medication","Helps diagnose Cushing's syndrome and distinguish its causes","Requires blood samples taken at specific time intervals","Involves taking dexamethasone pills for 2.5 days","Results help doctors determine if adrenal or pituitary issues are present"}
Key facts at a glance
- LOINC
- 50420-9
- Short code
- ACTH-LDDST
- Synonyms
- ACTH p 500 ug Dex Plas-mCnc
- Category
- chemistry
- Sample
- plasma
- Container
- EDTA plasma tube (Lavender top / Purple top), also use trasyloyl aprotinin tube to prevent ACTH degradation
- Units
- N/A
- Ref low
- 0.1
- Ref high
- 5
- Critical range
- — – 20 N/A
- Fasting
- false
- Population
- adult
- Sex
- all
- Age range
- 0–120 years
- Method
- Chemiluminescence Immunoassay (CLIA) or Electrochemiluminescence Immunoassay (ECLIA)
- CPT
- 82533
- SNOMED
- 413618004
- Specimen volume
- 1-2 mL of plasma
- Stability
- Room temperature: 1 hour; Refrigerated (2-8°C): 24 hours; Frozen (-20°C or below): 2-3 months. Avoid repeated freeze-thaw cycles. Transport on ice.
- Hemolysis max
- 50
- Lipemia max
- 100
- Icterus max
- 60
- TAT
- 24–72 hours
- Order
- routine
This is a laboratory test that measures specific substances in a sample of your blood to help your healthcare provider evaluate your overall health, screen for certain conditions, or monitor a known medical problem over time.
The ACTH stimulation test with low-dose dexamethasone is a diagnostic procedure that measures how your pituitary gland responds to cortisol suppression. After taking low-dose dexamethasone (a synthetic steroid) for 2.5 days, your blood is drawn to measure ACTH levels. This test helps doctors understand how your adrenal glands and pituitary gland are functioning together.
A trained professional will clean the skin on your arm, draw a small sample of blood from a vein using a thin needle, and send the sample to a laboratory, where specialized equipment analyzes it and your provider reviews the results.
You will take dexamethasone pills (0.5 mg) every 6 hours for 2.5 days (a total of 10 doses). After the last dose, blood samples will be collected to measure your ACTH levels. The test is usually done in the morning, and you may need to stay at the clinic for the final blood draw.
This test gives your healthcare provider objective information about how your body is functioning, which helps confirm or rule out a diagnosis, guide treatment decisions, and track how well a condition is responding to therapy.
This test is crucial for diagnosing Cushing's syndrome, a condition caused by excessive cortisol levels. It helps distinguish between different causes: whether the problem originates from the pituitary gland (Cushing's disease), adrenal glands, or from a tumor elsewhere in the body. This differentiation is essential for determining the correct treatment approach.
The risks are minimal. You may feel a quick sting when the needle is inserted, and mild bruising, soreness, or slight bleeding at the puncture site can occur. Serious complications such as infection are rare, and most people return to normal activities right away.
The risks are minimal and include minor bruising at the injection site, dizziness, or rare instances of infection. The low-dose dexamethasone used is generally well-tolerated, but some patients may experience mild side effects like nausea, headache, or difficulty sleeping. Contact your doctor if you experience severe symptoms.
Reference range
Suppressed: <5 pg/mL (<1.1 pmol/L) suggests normal hypothalamic-pituitary-adrenal axis suppression. Incomplete suppression (≥5 pg/mL) may indicate Cushing's disease or ectopic ACTH production.N/A
Patient must receive dexamethasone 0.5 mg (500 μg) orally every 6 hours for 2.5 days (6 doses). Take with food to minimize GI upset. Last dose given 6 hours before blood collection. Emphasize importance of compliance with medication schedule. Document exact timing of dexamethasone administration. Avoid stress, caffeine, and exercise before sample collection.
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. In normal responses, low-dose dexamethasone should suppress ACTH production. If ACTH remains elevated, it suggests ACTH-dependent Cushing's disease (usually pituitary tumor). If ACTH is suppressed, it suggests ACTH-independent Cushing's (usually adrenal tumor). The test also helps monitor treatment effectiveness and evaluate adrenal gland function.
Frequently asked questions
Do I need to fast before this test?
How long does the test take?
What do abnormal results mean?
Should I stop my regular medications?
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 15, 2026. · Read our editorial policy