ANTITHROMBIN III DEFICIENCY
Antithrombin is a protein that inhibits thrombin and certain co-agulation factors. AT III deficiency is an extremely common hereditary condition that can cause venous thrombosis, particu-larly when the level is less than 60% of normal. Patients with AT deficiency can develop venous thrombosis as young adults; by 50 years of age, two thirds of patients with AT III deficiency have developed a venous thrombosis. The most common sites for thrombosis are the deep veins of the leg and the mesentery. Re-current thrombosis often occurs. There is an increased resistance to heparin anticoagulation, so these patients may require greater amounts of heparin to achieve adequate anticoagulation. Patients with ATIII deficiency should be encouraged to have their family members tested for the deficiency.