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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 abnormal blood clots from forming.

Quick summary

["Antithrombin III helps maintain healthy blood flow by regulating clotting processes."]

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

Key facts at a glance

LOINC
8058-0
Short code
AChR Ab Ser-aCnc
Synonyms
AChR Ab Ser-aCnc
Category
coagulation
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
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 recommend this test if you have a history of blood clots, bleeding disorders, or if you’re taking medications that affect blood clotting. It could also be part of a general health check or evaluation for conditions like protein C or S deficiency.

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 lab uses a specific protocol. Wear a loose shirt for easy access to your arm.

How the Test Works

A small amount of blood is drawn from a vein in your arm and sent to a lab. The lab measures how well antithrombin III can inhibit clotting activity, often using an immunoassay or similar method.

What the Results Mean

Results are typically reported as a percentage or ratio compared to a standard. Low levels may indicate an increased risk of clotting, while very low levels could suggest a bleeding disorder or medication side effect.

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?
Several factors can lower antithrombin III, including certain medications (like heparin or warfarin), liver disease, kidney problems, or conditions that affect protein synthesis in the body.
What happens if my results are low?
If your levels are low, your doctor may recommend adjustments to your medications, lifestyle changes (like staying active), or further testing to identify the underlying cause.
Is this test painful?
Most people describe the blood draw as mild discomfort, similar to a needle stick. The area may feel sore afterward, but it usually goes away quickly.
  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