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Antithrombin III (ATIII) Activity, Plasma

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

This test measures the activity 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 maintains healthy circulation."]

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

Key facts at a glance

LOINC
105037-6
Short code
OXA-23 Islt/Spm Ql IA.rapid
Synonyms
OXA-23 Islt/Spm Ql IA.rapid
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-rapid
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, bleeding disorders, or if you’re taking medications that affect blood clotting. It could also be used to monitor treatment for conditions like disseminated intravascular coagulation (DIC) or to check for inherited clotting disorders.

What to Expect Before the Test

No special preparation is usually needed, but you may be asked to fast for a few hours before the blood draw. Avoid smoking or consuming alcohol for at least 24 hours prior. If you’re on blood-thinning medications, let your provider know before the test.

How the Test Works

A small amount of blood is drawn from a vein in your arm. The blood sample is then tested in a lab to measure the activity of antithrombin III, which helps control blood clotting processes.

What the Results Mean

Results are typically reported as a percentage or ratio of normal levels. Low levels of antithrombin III may indicate an increased risk of clotting, while very low levels could suggest a bleeding disorder or other underlying health issue.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Clinical Significance

Short name

Antithrombin III Level

Frequently asked questions

What are the normal reference ranges for this test?
Normal reference ranges can vary slightly between labs, but generally, antithrombin III activity is considered within the range of about 70% to 130% of normal. If your results fall outside this range, further evaluation may be needed to determine the cause.
How long does it take to get my results?
Test results are usually available within 1 to 3 business days, depending on your lab’s processing schedule. Your healthcare provider will discuss the results with you during your follow-up visit.
Can I eat or drink anything before the blood draw?
You may eat a normal meal before the blood draw, but it’s best to avoid heavy meals or alcohol for a few hours beforehand. Follow your healthcare provider’s specific instructions for optimal test accuracy.
  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