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Antithrombin III Activity (Serum)

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

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

Quick summary

["Antithrombin III helps regulate blood clotting; low levels may increase clot risk."]

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

Key facts at a glance

LOINC
43313-6
Short code
FAdV127 Ab Titr Ser HAI
Synonyms
FAdV127 Ab Titr Ser HAI
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
hai
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 Is Done

Your doctor may order this test if you have a history of blood clots, certain blood disorders, or conditions like liver disease or kidney problems that affect protein production. It might also be checked before surgery or if you’re taking medications that thin your blood.

What to Expect Before the Test

No special preparation is needed, but avoid eating or drinking anything for 8–12 hours before the draw. You may feel a slight pinch when the blood is taken from your arm.

How the Test Works

A small amount of blood is drawn from a vein, usually in your arm. The sample is sent to a lab where it’s tested for antithrombin III activity using a specialized method to measure its ability to inhibit clotting.

Possible Results and Next Steps

Results are reported as a percentage of normal activity. If levels are low, your doctor may recommend treatments like blood thinners or lifestyle changes to reduce clot risk.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

AT III Activity

Frequently asked questions

What does a low antithrombin III level mean?
A low result suggests your body may struggle to control blood clotting effectively, which could raise risks for clots in veins or arteries. Your doctor will discuss next steps, such as monitoring or treatment.
Can I eat or drink before this test?
Yes, but avoid food and liquids for 8–12 hours before your blood draw to ensure accurate results.
Is this test painful?
Most people feel only a brief pinch when blood is drawn, though some may experience mild discomfort. Most people recover quickly afterward.
  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