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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 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
110803-4
Short code
AChR Ab Msmt Ser
Synonyms
AChR Ab Msmt Ser
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 order this test if you have symptoms of blood clots (such as swelling, pain, or redness in the legs) or if you have a family history of clotting disorders. It may also be done before surgery to assess your risk of bleeding or clotting. Rarely, it could be checked if you have unexplained bleeding or if you’re taking medications that affect blood clotting.

What to Expect Before the Test

No special preparation is needed. You may have a small needle inserted into your arm to draw blood. If you’re taking blood thinners, let your provider know before the test.

What the Results Mean

Low levels of antithrombin III can increase your risk of blood clots, while very low levels may raise concerns about bleeding. Your results will be compared to a standard range for this test.

Common Conditions Linked to Results

Abnormally low levels of antithrombin III can be linked to inherited clotting disorders (like antithrombin deficiency) or conditions that cause the liver to produce fewer proteins, such as liver disease.

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 levels can be caused by genetic mutations, liver disease, severe infections, or certain medications that affect protein production. If you have symptoms of clotting or bleeding, this test helps identify whether your levels are abnormal.
How often should I get this test?
Your provider will determine how often you need this test based on your medical history, symptoms, and treatment plan. Some people may need monitoring more frequently if they have a clotting disorder or take blood-thinning medications.
Can I eat or drink before this test?
You can eat and drink normally before the test, as no special diet is required. However, let your provider know if you’re taking any medications that might affect blood clotting.
  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