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Antithrombin III Activity (Plasma)

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 helps regulate blood clotting and may be checked if you have a history of clotting disorders or bleeding problems."]

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

Key facts at a glance

LOINC
104474-2
Short code
Y pseudoTB 3 Ab Stl Ql
Synonyms
Y pseudoTB 3 Ab Stl Ql
Category
chemistry
Sample
stool
Container
Clean container
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
5 g
Stability
Refrigerated: 24h, Frozen: 7d
TAT
4–24 hours
Order
routine

Why This Test Might Be Ordered

Your healthcare provider may recommend this test if you have symptoms like easy bruising, excessive bleeding, or a history of blood clots. It could also be part of a screening or monitoring plan for conditions like thrombophilia or liver disease.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for a few hours before the blood draw. Avoid smoking or consuming caffeine for 30 minutes before the test if instructed.

How the Test Works

A small amount of blood is drawn from a vein in your arm. The blood sample is then analyzed to measure antithrombin III activity, which helps assess its ability to inhibit blood clotting.

What the Results Mean

Results are typically reported as a percentage or ratio compared to a standard. Low levels may indicate a higher risk of bleeding, while very low levels could suggest a clotting disorder or liver issues.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Antithrombin III Activity

Frequently asked questions

What causes low antithrombin III levels?
Low antithrombin III can be caused by liver disease, pregnancy, certain medications (like heparin), or inherited conditions like antithrombin deficiency. It may also be temporary due to trauma, surgery, or severe infections.
How long does it take to get results?
Results are usually available within 1 to 3 business days, depending on your lab’s processing time and any follow-up tests needed.
Can I eat or drink before this test?
Check with your healthcare provider, but generally, no food or drink (except water) is required unless specified otherwise.
  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