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Antithrombin III Activity (Antithrombin Concentration)

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
81853-4
Short code
Chr 15 + 16 + 22 aneu Bld/T
Synonyms
Chr 15 + 16 + 22 aneu Bld/T
Category
chemistry
Sample
whole-blood
Container
Lavender-top (EDTA)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
molgen
Specimen volume
3 mL
Stability
Room temperature: 4h, Refrigerated: 24h
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

Why Is This Test Done?

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 liver disease or cancer.

What Does a Result Mean?

Low levels of antithrombin III can increase your risk of clotting, while very low levels may suggest bleeding problems. Your results will be compared to a general reference range provided by your lab.

How Is the Test Performed?

A blood sample is drawn from a vein, usually in your arm. The sample is processed to measure antithrombin III activity, often using an immunoassay method.

What Are Common Causes of Abnormal Results?

Conditions like liver disease, kidney problems, or certain medications (e.g., heparin) can affect antithrombin III levels. Inherited deficiencies are also possible.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Antithrombin III Activity

Frequently asked questions

What happens if my antithrombin III level is low?
A low level may mean you’re at higher risk of blood clots. Your doctor may recommend blood thinners, lifestyle changes, or further testing to understand the cause.
How often should I get this test?
Your doctor will determine how often you need retesting based on your medical history, treatment plan, or symptoms. Follow their guidance closely.
Can I eat or drink before this test?
Typically, you may fast for 8–12 hours before the test, but always check with your lab or doctor for specific instructions.
  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