(See also Overview of Thrombotic Disorders.)
Because antithrombin inhibits thrombin and factors Xa, IXa, and XIa, deficiency of antithrombin predisposes to venous thrombosis.
Antithrombin is a protein that inhibits thrombin and factors Xa, IXa, and XIa, thereby inhibiting thrombosis.
Heterozygous deficiency of plasma antithrombin has a prevalence of about 0.2 to 0.4%; about half of people affected develop venous thromboses. Homozygous deficiency is probably lethal to the fetus in utero.
Acquired deficiencies occur in patients with disseminated intravascular coagulation, liver disease, or nephrotic syndrome, or during heparin therapy. Heparin exerts its anticoagulant effect by activating antithrombin.
Laboratory testing is done for patients with an unexplained blood clot and involves quantification of the capacity of patient plasma to inhibit thrombin in the presence of heparin.