Factor XI is a protein which plays an important role in blood clotting. This test is used to measure the amount of factor XI in one’s blood. By not meeting with a sufficient amount of factor XI, the person may develop factor XI deficiency. This deficiency is a bleeding disorder also known as hemophilia C. Due to this deficiency the clotting reaction is blocked and the blood clot is not formed. This test is needed when a doctor may see symptoms of factor XI deficiency. A platelet count may also be done which is always done in case of any bleeding disorder.