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Antithrombin III Activity (Serum)

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

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

Quick summary

["Antithrombin III helps maintain healthy blood flow by inhibiting clotting factors."]

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

Key facts at a glance

LOINC
56638-0
Short code
Deprecated RA Nuc Ab Titr Ser Ql IF
Synonyms
Deprecated RA Nuc Ab Titr Ser Ql IF
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
if
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 order this test if you have a history of blood clots, bleeding disorders, or if you’re taking medications that affect blood clotting. It may also be checked before surgery or during treatment for conditions like disseminated intravascular coagulation (DIC) or liver disease.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for 8–12 hours before the draw if your blood is collected in a specific tube. Avoid aspirin or certain blood thinners for a few days before testing, as they can affect results.

How the Test Works

A small sample of your blood is drawn and sent to a lab. The test measures how well antithrombin III can block clotting proteins, often using a chromogenic assay that detects color changes to quantify activity levels.

What the Results Mean

Low levels of antithrombin III may suggest conditions like liver disease, protein-losing syndromes, or inherited clotting disorders. Very low levels can increase your risk of clots. High levels may be seen in certain genetic conditions or during pregnancy.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Antithrombin III Activity

Frequently asked questions

What causes antithrombin III levels to be low?
Low antithrombin III levels can result from inherited genetic mutations, liver disease (which reduces protein production), severe infections, or conditions that cause excessive protein loss (like kidney disease). Certain medications or blood clotting disorders may also play a role.
How long does it take to get results?
Results are typically available within 1–3 business days, depending on your lab’s processing time. Your healthcare provider will discuss the findings with you during your follow-up visit.
Can I take anything to help raise my antithrombin III levels?
There’s no specific diet or supplement to boost antithrombin III levels, but managing underlying conditions like liver disease or kidney problems can help. If you have a genetic deficiency, your provider may recommend clotting medications to prevent clots.
  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