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. 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 DIC, liver disease, or nephrotic syndrome and during heparin or l-asparaginase therapy. (The inhibitory activity of antithrombin is greatly augmented by heparin.)

Laboratory testing involves quantification of plasma inhibition of thrombin in the presence of heparin.

Oral warfarin is used for prophylaxis against venous thromboembolism.

Last full review/revision January 2013 by Joel L. Moake, MD