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Antithrombin III (ATIII) Activity in Serum

Published: August 9, 2026Updated: August 17, 2026

This test measures the activity level of antithrombin III, a natural protein in your blood that helps prevent abnormal blood clotting.

Quick summary

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

Medically reviewed by WellAlly Editorial TeamLast reviewed August 17, 2026Editorial policy

Key facts at a glance

LOINC
50732-7
Short code
SRA 0.1U/mL LMW heparin Ser-a
Synonyms
SRA 0.1U/mL LMW heparin Ser-a
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
0
Ref high
17
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, certain blood disorders, or are taking medications that affect clotting. It may also be checked before surgery or if you experience unexplained bleeding.

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 requires it. Wear a loose shirt for easy access to your arm.

How the Test Works

A blood sample is taken from a vein in your arm. The serum (liquid part of blood after clotting) is then analyzed to measure antithrombin III’s ability to inhibit clotting proteins.

What the Results Mean

Results are reported as a percentage or ratio of normal levels. Very low results may indicate a clotting disorder, while very high results may suggest liver disease or other conditions.

Reference range

NCBI: NSE 0-17 ng/mLN/A

Specimen: serum

No special preparation usually required. Stay well hydrated. Inform your healthcare provider of all medications and supplements you are taking.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Antithrombin III Level

Frequently asked questions

Antithrombin III deficiency is linked to an increased risk of venous thromboembolism (clots in veins) or arterial clots. It’s also checked in patients on anticoagulant therapy to monitor treatment effectiveness.
Antithrombin III is a protein that normally keeps blood from clotting too much. If your levels are too low, your body may form clots more easily, especially if you have risk factors like obesity, smoking, or a family history of clotting disorders.
How long does it take to get results?
Results are typically available within 1–3 business days, depending on your lab. Follow-up with your provider to discuss next steps if needed.
What causes low antithrombin III levels?
Low levels can result from genetic mutations, liver disease, pregnancy, or certain medications. Rarely, it may be due to autoimmune conditions or severe infections.
  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 17, 2026. · Read our editorial policy