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Serum or plasma concentration of antithrombin III (ATIII)

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 supports blood circulation by regulating clotting processes."]

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

Key facts at a glance

LOINC
53018-8
Short code
ENA SS-a 60kD Ab Ser Ql
Synonyms
ENA SS-a 60kD Ab Ser Ql
Category
chemistry
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

What Is Antithrombin III?

Antithrombin III is a natural blood protein that acts as an inhibitor of several clotting factors. It helps maintain a balance between blood clotting and fluidity, preventing excessive clotting that could lead to strokes, heart attacks, or other serious conditions. Low levels may increase your risk of clotting disorders.

Why Might You Need This Test?

Your doctor may order this test if you have a history of blood clots, are taking blood thinners, have certain genetic conditions (like antithrombin deficiency), or are undergoing treatments that affect clotting, such as chemotherapy. It can also be checked before surgeries or during pregnancy to assess clotting risks.

How Is the Test Performed?

A blood sample is drawn from a vein, usually in your arm. The sample is sent to a lab where it is analyzed to measure the concentration of antithrombin III. The test can be done on serum (blood after clotting) or plasma (blood after anticoagulant treatment).

What Do Results Mean?

Results are typically reported as a percentage or ratio compared to a standard. Very low levels may indicate a higher risk of clotting, while very high levels might suggest underlying conditions like liver disease or certain medications. Your doctor will interpret results in the context of your medical history.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Antithrombin III

Frequently asked questions

How often will I need this test?
The frequency depends on your medical condition. If you have a known antithrombin deficiency or are on blood-thinning treatment, your doctor may recommend regular monitoring. Otherwise, it may be done as needed for specific situations, such as before surgery or during pregnancy.
What are the normal ranges?
There is no single standard range for everyone. Reference ranges can vary by lab and may be adjusted based on your health status. Your doctor will provide personalized guidance based on your test results.
Can I eat or drink before the test?
For most blood tests, you can eat and drink normally unless instructed otherwise by your doctor. However, if you’re taking any medications that affect clotting, check with your healthcare provider before the test.
  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