Acthpre500ugdexplasmcnc

Published: August 9, 2026Updated: August 10, 2026

This test measures the ACTH hormone in your blood before and after taking a low dose of dexamethasone to help diagnose causes of cortisol excess.

Quick summary

{"Measures ACTH (corticotropin) hormone levels in blood plasma","Involves taking a low-dose dexamethasone pill and having blood drawn at specific times","Helps doctors determine if you have Cushing's syndrome and identify its cause","Requires multiple blood draws over 2-3 days","No fasting required, but follow your doctor's instructions carefully"}

Medically reviewed by WellAlly Editorial TeamLast reviewed August 10, 2026Editorial policy

Key facts at a glance

LOINC: 50419-1
Short code: ACTH
Synonyms: ACTH pre 500 ug Dex Plas-mCnc
Category: chemistry
Sample: plasma
Container: Lavender-top (EDTA) plastic plasma tube or pink-top (K2EDTA) tube
Units: N/A
Ref low: 10
Ref high: 60
Critical range: 5 N/A – 200 N/A
Fasting: false
Population: adult
Sex: all
Age range: ≥ 18 years
Method: Chemiluminescent Immunoassay (CLIA) or Electrochemiluminescence Immunoassay (ECLIA)
CPT: 82024
SNOMED: 313295005
Specimen volume: 1.0-2.0 mL plasma minimum
Stability: Room temperature: 2-4 hours; Refrigerated (2-8°C): 24-48 hours; Frozen (-20°C or below): 2-3 months. Avoid repeated freeze-thaw cycles.
Hemolysis max: 50
Lipemia max: 300
Icterus max: 20
TAT: 24–72 hours
Order: routine

What is this test?

The ACTH test with low-dose dexamethasone suppression measures the level of adrenocorticotropic hormone (ACTH) in your blood. ACTH is a hormone produced by the pituitary gland that tells your adrenal glands to make cortisol, the stress hormone. This test is used to evaluate how your body responds to dexamethasone, a medication that mimics cortisol's effects on the body.

How is the test performed?

The test typically takes place over 2-3 days. First, a baseline blood sample is taken to measure your ACTH and cortisol levels. Then, you will take a low dose of dexamethasone pills (usually every 6 hours) for 2-3 days. Additional blood samples will be drawn at specific times to see how your ACTH levels respond. The timing and number of blood draws may vary based on your doctor's orders.

How to prepare for the test

Follow your doctor's instructions exactly regarding when to take the dexamethasone pills. Take them at the exact times prescribed, with or without food as directed. Inform your doctor about all medications you are taking, as certain drugs can affect test results. Do not stop taking any prescription medications unless your doctor tells you to. Tell your doctor if you are pregnant or have any known adrenal or pituitary conditions.

What will I feel during the test?

The blood draw may cause brief discomfort, bruising, or dizziness. Taking dexamethasone may cause mild side effects such as nausea, headache, or difficulty sleeping. These side effects are usually temporary. If you experience severe symptoms, contact your healthcare provider.

When will I get the results?

Test results are usually available within a few days to a week, depending on the laboratory. Your doctor will discuss the results with you and explain what they mean for your diagnosis and treatment plan.

Reference range

Morning (8 AM) baseline: 10-60 pg/mL (2.2-13.2 pmol/L). Reference ranges may vary by laboratory methodology. Post-dexamethasone suppression: <5 pg/mL suggests normal cortisol suppression.N/A

Specimen: plasma

Patient undergoes low-dose dexamethasone suppression test (500 mcg PO every 6 hours for 2.5 days). This pre-dexamethasone sample is collected BEFORE initiating dexamethasone administration. Patient should fast overnight (8-12 hours) but may drink water. Avoid glucocortico medications for 24-48 hours prior unless specifically instructed. Inform about any recent steroid use, stress, or illness as these affect results.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Frequently asked questions

Why do I need to take dexamethasone for this test?

Dexamethasone is a synthetic steroid that acts like cortisol. In a normal pituitary-adrenal system, taking dexamethasone should suppress ACTH production. By seeing how your ACTH levels respond to dexamethasone, doctors can determine if your cortisol regulation system is working properly and identify the cause of any abnormalities.

What happens if my ACTH is not suppressed?

If your ACTH levels do not decrease after taking dexamethasone, it may indicate that your pituitary gland is producing too much ACTH (suggesting Cushing's disease), or that an ectopic tumor is producing ACTH. Your doctor will interpret these results in context with your other test results and clinical symptoms.

Can I take my other medications during this test?

You should inform your doctor about all medications you are taking before the test. Certain medications, such as corticosteroids, birth control pills, epilepsy drugs, and some antidepressants, can affect the test results. Your doctor will tell you which medications to avoid or adjust before and during the test.

Is this test safe?

The low-dose dexamethasone suppression test is generally safe for most patients. However, dexamethasone can interact with certain conditions. Tell your doctor if you have diabetes, high blood pressure, stomach ulcers, or if you are pregnant. The small amount of dexamethasone used in this test is unlikely to cause significant side effects in most people.

Do I need to fast before this test?

Fasting is typically not required for this test. However, follow the specific instructions provided by your healthcare provider, as they may have given you personalized directions based on your situation.

Sources

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This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

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