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Antithrombin III (ATIII) activity, plasma

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. It’s important for managing conditions that increase your risk of clotting.

Quick summary

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

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

Key facts at a glance

LOINC
50197-3
Short code
aPTT Pnl PPP
Synonyms
aPTT Pnl PPP
Category
coagulation
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
0.8
Ref high
1.2
Critical range
— – 5 N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
CPT
85610
SNOMED
396252006
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

Your doctor may order this test if you have a history of blood clots, certain blood disorders, or conditions that affect protein synthesis (like liver disease). It can also be checked before surgery or if you’re taking blood-thinning medications to ensure they’re working properly.

What to Expect Before the Test

No special preparation is needed, but you may be asked to fast for a few hours before your blood is drawn. The test involves a simple venipuncture (blood draw) from a vein in your arm.

What the Results Mean

Results are typically reported as a percentage or ratio compared to a standard. Very low levels may indicate conditions like liver disease, protein-losing syndromes, or inherited clotting disorders. Very high levels are less common but could suggest certain medications or rare genetic factors.

Common Conditions Linked to Results

Low antithrombin III levels are often linked to an increased risk of blood clots (thrombosis) or disseminated intravascular coagulation (DIC). High levels may be associated with certain medications or rare genetic variations.

Reference range

INR: 0.8-1.2 (warfarin: 2.0-3.0)N/A

Specimen: serum

Inform your healthcare provider if you are taking blood thinners. No special preparation required.

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 antithrombin III levels can be caused by inherited genetic mutations, liver disease (which affects protein production), certain medications, or conditions that cause protein loss in urine or stool (like nephrotic syndrome).
How is antithrombin III tested?
The test measures your plasma level of antithrombin III using a specialized immunoassay method. A small amount of blood is drawn, and the lab analyzes it to determine your activity level compared to a standard.
Can antithrombin III levels change with diet or exercise?
Generally, antithrombin III levels are not significantly affected by diet or exercise. However, conditions like liver disease or certain medications may influence them, so it’s important to follow your doctor’s advice regarding lifestyle factors if you have a known condition.
  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