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Antithrombin III Activity (Serum or Plasma)

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

This test measures the level of antithrombin III, a natural 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
91138-8
Short code
ITPA g.9381A>C SerPl
Synonyms
ITPA g.9381A>C SerPl
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
molgen
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

Doctors may order this test to evaluate blood clotting function, especially if you have conditions like deep vein thrombosis (DVT), pulmonary embolism, or inherited bleeding disorders. It can also be used to monitor treatment for conditions affecting blood clotting or protein synthesis.

What to Expect Before the Test

No special preparation is needed. You may have blood drawn from a vein in your arm. If you’re on anticoagulant medications, inform your healthcare provider before the test.

What the Results Mean

Low levels of antithrombin III may indicate an increased risk of blood clots, while very low levels could suggest liver disease or other conditions affecting protein production. Results are interpreted by your healthcare provider in the context of your medical history.

Common Conditions Monitored

This test is often used to diagnose or monitor conditions like antithrombin deficiency, liver disease, or to assess the effectiveness of treatments for clotting disorders.

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 levels can result from inherited genetic mutations, liver disease, severe infections, or certain medications that affect protein synthesis. Lifestyle factors like smoking or obesity may also play a role.
How long does it take to get results?
Results are typically available within 1–3 business days, depending on your lab’s processing time. Your healthcare provider will discuss them with you during your follow-up visit.
Can I eat or drink before this test?
You can eat and drink normally before the test unless instructed otherwise by your doctor. Avoiding food or fluids for extended periods is not required for this 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