- permanent risk factors for venous thromboembolism: patients known to have antiphospholipid antibodies or lupus anticoagulant (based on local laboratory criteria) or to have homozygous factor V Leiden or homozygous prothrombin G21210A or heterozygous factor V Leiden plus heterozygous prothrombin G21210A or antithrombin III deficiency; patients with active malignancy