Aspirin for the Prevention of Recurrent Venous Thromboembolism

Inclusion Criteria:

• first episode of symptomatic, objectively confirmed idiopathic proximal deep vein thrombosis and/or pulmonary embolism;
• initial treatment with unfractionated heparin or low-molecular-weight heparin (or effective alternative) followed by a vitamin K antagonist (target INR 2.0-3.0). All patients will receive 6 or 12 months of oral anticoagulant treatment. Patients initially treated with thrombolytic therapy who received warfarin therapy are eligible for inclusion.

Exclusion Criteria:

• 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
• temporary risk factors for venous thromboembolism
• any recurrence of venous thromboembolism or bleeding episode during the established 6-month period of oral anticoagulant treatment
• allergy or intolerance of aspirin
• clear indication for aspirin or other anti-platelet therapy (e.g. clopidogrel, ticlopidine)
• clear indication for long-term anticoagulant therapy (e.g. recurrent idiopathic venous thromboembolism, prosthetic heart valve)
• treatment with non-selective COX-1/2 non-steroidal anti-inflammatory drugs
• life expectancy less than 6 months
• active bleeding or at high risk of bleeding (gastrointestinal bleeding within the past 12 months; endoscopic diagnosis of peptic ulcer disease or ulcerative esophagitis within the past 6 months unless there is documented endoscopic evidence of healing; intracranial bleeding within the past year; known bleeding diathesis)
• anticipated non-adherence to study medications
• inability to attend follow up because of geographic inaccessibility
• failure to provide informed consent