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Serum or Plasma Immunoassay for Antithrombin III Activity (Qualitative or Quantitative)

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

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

Quick summary

["Antithrombin III helps maintain healthy blood flow by inhibiting clotting factors."]

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

Key facts at a glance

LOINC
29901-6
Short code
HTLV I+II Ab Ser Ql IA
Synonyms
HTLV I+II Ab Ser Ql IA
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
ia
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, are taking blood thinners, or have conditions like liver disease that affect protein production. It could also be used to monitor treatment for conditions linked to clotting disorders.

How the Test Works

Blood is drawn and analyzed to detect the presence or amount of antithrombin III. The test may be qualitative (yes/no) or quantitative (specific concentration), depending on clinical needs.

Sample Collection and Preparation

A blood sample is taken from a vein, typically in your arm. The blood is processed to separate serum (liquid part after clotting) or plasma (liquid part before clotting). This sample is then sent to a lab for testing.

Test Results Interpretation

Results are compared to expected levels for healthy individuals. Low levels may indicate conditions affecting clotting or liver function, while normal levels suggest healthy protein function.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Antithrombin III (ATIII) Test

Frequently asked questions

What does a low level of antithrombin III mean?
Low levels can signal conditions like liver disease, disseminated intravascular coagulation (DIC), or inherited clotting disorders. It may also be checked if you’re on anticoagulant therapy to ensure it’s working properly.
What does a high level of antithrombin III mean?
High levels are less common but may occur in certain genetic conditions or during pregnancy. Your provider will discuss results in the context of your overall health.
How often should I get this test?
Your provider will determine the frequency based on your medical history, treatment plan, or symptoms. Regular monitoring may be needed if you have ongoing clotting risks or liver issues.
  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