Abstract
Factor XI (FXI) is the zymogen precursor of an active serine protease that participates in the contact phase of coagulation. Synthesized in the liver, it circulates in the plasma in a noncovalent complex with high molecular weight kininogen (1) at a normal concentration of 5 μg/mL. (For clinical purposes, the normal range is defined as 50-150 U/dL) (2). FXI circulates as a homodimeric glycoprotein with a mass of 160 kDa.