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The American College of Chest Physicians recommends antithrombotic therapy for at least 3 months after a first unprovoked venous thromboembolism (VTE, deep venous thrombosis or pulmonary embolism).  At the end of 3 months, the recommendation is to either continue or discontinue therapy dependent on the benefit versus risk of continuing antithrombotic therapy.1 Extending therapy can reduce the risk of recurrent VTE by 264 events per 1000 patient at 5 years, however it also increases the risk of a major bleed by 24 to 98 per 1000 patients (with 3 to 11 fatal events) depending on the patient’s baseline risk of bleeding. Extending antithrombotic therapy with warfarin also means at least monthly monitoring and drug and diet interactions.1 In individuals who stop antithrombotic therapy, other alternatives to prevent VTE with a lower risk of bleeding and less monitoring and interactions would be desirable. Antiplatelet agents, such as aspirin, have been shown to prevent arterial thrombosis by preventing platelet aggregation.2,3 Aspirin has been shown to prevent cardiovascular events, such as stroke and myocardial infarction (MI) in patients with and without cardiovascular disease(CVD).2 Brighton and colleagues sought to determine if low-dose aspirin could prevent VTE in patients after antithrombotic therapy for an initial unprovoked VTE.3

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The Aspirin to Prevent Recurrent Venous Thromboembolism (ASPIRE) trial was a randomized, double-blind, placebo-controlled, multi-center study that randomized 822 patients with an initial unprovoked VTE who had completed a trial of warfarin therapy to aspirin 100mg daily or placebo for up to 4 years. Individuals completed 6 weeks to 24 months of warfarin at an INR of 2-3, and were not eligible if the VTE was more than 2 years prior or if they had an indication for aspirin, another antiplatelet agent, non-steroidal anti-inflammatory drugs or anticoagulation therapy. The primary endpoint was a recurrent VTE with secondary endpoints of major vascular events (composite of VTE, MI, stroke or CV death) and a measure of net clinical benefit (major vascular events plus major bleed or death from any cause).3 Due to problems with meeting the initial sample size and slow recruitment, the investigators planned to pool the results with the Warfarin and Aspirin (WARFASA) trial that had similar trial design and methods.3,4

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Of the 822 patients enrolled in the study, 54% (n=447) were men and the median age was 54 years. The patient characteristics were not significantly different between the groups. The initial event was a proximal DVT in 57%, a pulmonary embolism in 28% and a DVT and PE in 14%. The primary outcome of recurrent VTE occurred in 18% in the placebo arm and 14% with aspirin (rate of 6.5% and 4.8% per year, respectively; HR 0.74, 95% CI, 0.52-1.05, p=0.09). Of the recurrent events, 77% were unprovoked and 2 fatal PE occurred (one in each group). Risk of a recurrent event was highest in the first year and the effect of treatment was similar among subgroups. Major vascular events were significantly lower in the ...

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