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Antithrombin III Activity (Serum)

Published: August 9, 2026Updated: August 16, 2026

This test measures the level of antithrombin III, a natural protein in your blood that helps prevent abnormal blood clots from forming.

Quick summary

["Antithrombin III supports blood clot stability and is important for preventing excessive clotting."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
55441-0
Short code
Deprecated AT III Ag PPP
Synonyms
Deprecated AT III Ag PPP
Category
chemistry
Sample
plasma
Container
Lavender-top (EDTA) or Green-top (Heparin)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why This Test Might Be Ordered

Your healthcare provider may recommend this test if you have a history of blood clots, certain blood disorders, or conditions that increase your risk of clotting, such as kidney disease or long-term use of certain medications. It may also be used to monitor treatment for conditions affecting blood clotting.

What to Expect Before the Test

No special preparation is needed. You may have a small needle inserted into a vein in your arm to draw blood. The process is quick and usually painless. If you’re on blood-thinning medication, let your provider know before the test.

What the Results Mean

Lower than normal levels of antithrombin III can increase your risk of blood clots. Very low levels may require treatment to manage clotting risks. Your results will be compared to a standard range provided by your lab.

Common Conditions Monitored

This test helps diagnose or monitor conditions like disseminated intravascular coagulation (DIC), hereditary antithrombin deficiency, or kidney disease that can affect blood clotting. It’s also used to assess response to treatments like heparin.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Antithrombin III

Frequently asked questions

How often do I need this test?
The frequency depends on your health condition and treatment plan. If you have a known clotting disorder or are on anticoagulant therapy, your provider may schedule follow-up tests every few months or as needed.
Can I eat or drink before this test?
You can eat and drink normally unless instructed otherwise. Avoid smoking or caffeine before the test, as it may affect blood flow and test results.
What are the normal reference ranges?
Reference ranges vary by lab, but typically, antithrombin III activity is measured as a percentage of normal levels, often between 70% and 120%. Your specific range will be provided by your healthcare provider.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

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.

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 16, 2026. · Read our editorial policy