Factor XI

The Factor XI test measures the activity of factor XI, a clotting protein made in your liver that helps your blood form stable clots. It is part of the intrinsic pathway of coagulation and works together with several other proteins to stop bleeding after injury or surgery.

Your sample is usually a blood draw collected in a citrate tube and analyzed in a coagulation laboratory. Some labs measure activity, some measure the amount of the protein, and sometimes both are done to help find the cause of a bleeding tendency.

Low factor XI activity can be inherited or acquired and may increase the chance of bleeding, especially with dental work, surgery, or injuries involving tissues that easily bleed. The degree of bleeding does not always match the level, so your personal history and planned procedures matter when decisions are made.

Clinicians order this test when the activated partial thromboplastin time is prolonged, when there is a family history of factor XI deficiency, before procedures in people with possible bleeding disorders, or to investigate unexplained bruising or bleeding. Changes can also be seen with liver disease or conditions that consume clotting factors.

Occasionally, higher levels are seen with inflammation or other physiologic states. Your care team interprets the result in context with other clotting tests, your medications, and your clinical situation.

Your factor XI result is interpreted alongside your bleeding history, physical exam, and other lab tests such as the activated partial thromboplastin time and levels of other clotting factors. If the activity is lower than expected, your clinician may repeat the test, check the amount of the protein, or perform specialized studies to look for an inhibitor or an acquired cause.

If you have a deficiency, planning for dental work, childbirth, or surgery may involve a hematology team to reduce bleeding risk. Treatment plans are individualized and may include preventive measures or factor replacement products when appropriate. If a result is unexpectedly high or low, a repeat sample and review of medicines that affect clotting are common next steps.

Do not start or stop any medication on your own. Share all prescription drugs, over‑the‑counter products, and supplements with your care team so they can interpret the test accurately and advise you on safe follow‑up.