Hello, Welcome to HCM As your niece is suffering from anti-thrombin III deficiency,and also she has a blood clot,she needs anticoagulation. Warfarin-oral anticoagulant is the drug of choice. Nowadays,low molecular weight heparins like argatroban and antithrombin III concentrates are also tried for this disease. These drugs are given lifelong to protect from recurrent Venous Thromboembolic Episodes.These drug therapy should be monitored very closely,as there are many side effects from this. Also, i would suggest your niece to avoid a variety of precipitating factors, such as taking oral contraceptives or hormone replacements, that may precede the development of Venous Thromboembolic Episodes. Hope i have cleared your doubts. Thank you.