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Antithrombin III (Factor XIa Inhibitor) Activity in Plasma

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

This test measures how well your blood naturally prevents blood clots from forming by checking levels of antithrombin III, a protein that helps regulate clotting.

Quick summary

["Antithrombin III helps control blood clotting; low levels may increase risk of clots."]

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

Key facts at a glance

LOINC
33987-9
Short code
Heparin CF II Act/Nor PPP Chro
Synonyms
Heparin CF II Act/Nor PPP Chro
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
Method
chromo
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

What Does This Test Measure?

Antithrombin III is a protein produced by your liver that binds to and inactivates certain enzymes involved in blood clotting. When these enzymes (like Factor XIa) are active, they promote clot formation. This test evaluates the functional activity of antithrombin III in your plasma, helping determine its ability to inhibit clotting pathways.

Why Might You Need This Test?

Doctors may order this test if you have a history of blood clots, certain genetic conditions (like antithrombin deficiency), or if you’re taking medications that affect blood clotting (such as heparin). It can also be used to monitor treatment progress or assess risk before surgeries.

How Is the Test Performed?

A blood sample is drawn from a vein, usually in your arm. The plasma portion is then analyzed using a fluorescence immunoassay (fia) method to measure antithrombin III’s ability to neutralize clotting enzymes.

What Are Common Causes of Abnormal Results?

Low levels of antithrombin III may indicate inherited deficiencies, liver disease, severe infections, or conditions that increase clotting risk. High levels could rarely occur with certain medications or rare genetic factors.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Antithrombin III Activity

Frequently asked questions

Why do I need this test?
This test helps identify whether your blood’s natural clotting inhibitors are functioning properly. Low levels may increase your risk of clots, especially if you have a family history or are taking medications that thin your blood.
What do my results mean?
Results below normal may signal a need for preventive measures, such as blood thinners or lifestyle changes. Results above normal are less common but may warrant further investigation for underlying causes.
Can I eat or drink before this test?
You may eat and drink normally, but avoid alcohol for 24 hours before testing. No special preparation is needed beyond fasting if directed by your doctor.
  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