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Antithrombin III Activity (Serum/Plasma)

Published: August 9, 2026Updated: August 17, 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 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 17, 2026Editorial policy

Key facts at a glance

LOINC
101863-9
Short code
Comp Fact H Ab SerPl IA-aCnc
Synonyms
Comp Fact H Ab SerPl IA-aCnc
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Ref low
10
Ref high
60
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 Is Done

Your healthcare provider may order this test if you have symptoms of blood clots (like swelling, pain, or redness in a limb) or bleeding disorders. It may also be used to monitor treatment for conditions like disseminated intravascular coagulation (DIC) or to check for inherited clotting risks.

How the Test Works

A small blood sample is drawn and tested for antithrombin III activity using a method that detects its ability to inhibit clotting. The result is expressed as a percentage of normal levels.

Sample Collection and Preparation

Blood is collected in a special tube that prevents clotting. The sample is then processed to separate serum or plasma for testing.

What to Expect Before the Test

No special preparation is needed, but inform your provider about any medications, supplements, or recent illnesses that could affect results.

Reference range

NCBI: ACTH 10-60 pg/mLN/A

Specimen: serum

Typically collected in the morning (8-10 AM). Inform your provider if taking steroid medications.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Antithrombin III Activity

Frequently asked questions

What does a low antithrombin III level mean?
A low result may indicate an inherited deficiency, liver disease, or conditions that increase clotting risk. Your provider will discuss next steps based on your symptoms and medical history.
How long does it take to get results?
Results are typically available within 1–3 business days, depending on lab processing times.
Can I eat or drink before this test?
You can eat and drink normally unless instructed otherwise by your provider.
  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 17, 2026. · Read our editorial policy