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Factor XI Inhibitor (Antithrombin Activity) in Plasma

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

This test measures how much Factor XI inhibitor is present in your blood plasma. Factor XI plays a role in blood clotting, and inhibitors can affect how quickly blood clots form or dissolve.

Quick summary

["This test helps assess bleeding risks or clotting disorders linked to Factor XI antibodies."]

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

Key facts at a glance

LOINC
32635-5
Short code
Fact II Inhib PPP-aCnc
Synonyms
Fact II Inhib PPP-aCnc
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

What Does This Test Measure?

Factor XI is a protein involved in the blood clotting process. When Factor XI is blocked by antibodies (inhibitors), it can interfere with normal clotting mechanisms. This test specifically measures the activity of Factor XI in your plasma, checking if inhibitors are present.

Why Might You Need This Test?

Your doctor may order this test if you have unexplained bleeding, recurrent clots, or a history of bleeding disorders. It’s also used in patients with hemophilia or other clotting disorders to monitor Factor XI activity.

How Is the Test Performed?

A blood sample is drawn from a vein, usually in your arm. The plasma portion is then tested for Factor XI activity to detect inhibitors. Results are compared to a standard to determine levels.

What Do the Results Mean?

Results may indicate the presence of Factor XI inhibitors, which could suggest bleeding tendencies or altered clotting. Your doctor will interpret results in the context of your symptoms and medical history.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Factor XI Inhibitor

Frequently asked questions

How common are Factor XI inhibitors in the general population?
Factor XI inhibitors are rare but can occur in people with bleeding disorders or those taking certain medications. They may mimic or worsen bleeding symptoms, so early detection is important.
Can lifestyle changes help if I have Factor XI inhibitors?
While lifestyle changes like avoiding trauma or certain medications may help manage symptoms, treatment often focuses on medications (e.g., bypass products) to control bleeding. Always follow your doctor’s advice.
Do I need to prepare for this test?
No special preparation is needed, but avoid heavy exercise or medications that affect blood clotting 24 hours before the test.
  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