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Fluorescent Immunoassay for Antithrombin III Activity in 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; low levels may increase clot risk."]

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

Key facts at a glance

LOINC
3232-6
Short code
Fact XII Act/Nor PPP
Synonyms
Fact XII Act/Nor PPP
Category
chemistry
Sample
plasma
Container
Lavender-top (EDTA) or Green-top (Heparin)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
coag
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 used to monitor treatment for conditions like disseminated intravascular coagulation (DIC) or hereditary antithrombin deficiency.

What to Expect Before the Test

No special preparation is needed. You’ll typically provide a blood sample drawn from a vein in your arm. If you’re on anticoagulant medications, inform your provider beforehand.

How the Test Works

Blood is drawn and processed to separate plasma. The test uses a fluorescent immunoassay to detect and quantify antithrombin III activity, which is compared to a standard reference.

What the Results Mean

Results are reported as a ratio or percentage relative to a standard. Low levels may indicate an increased risk of clotting, while normal levels suggest balanced clotting regulation.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance: Antithrombin III plays a critical role in preventing excessive clotting. Deficiencies can lead to recurrent clots, while elevated levels may be associated with certain clotting disorders or medications.

Short name

Antithrombin III (ATIII) Fluorescence Assay

Frequently asked questions

What causes antithrombin III levels to be low?
Low antithrombin III levels can result from genetic disorders, liver disease, pregnancy, or certain medications that affect clotting. Infections or severe stress can also temporarily reduce levels.
How long does it take to get results?
Results are typically available within 24 to 48 hours, depending on lab processing times and your healthcare provider’s workflow.
Can I eat or drink before this test?
You can eat and drink normally before this test, as no fasting is required. However, inform your provider if you’re on blood-thinning medications.
  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