Aspirin in the secondary prevention of unprovoked thromboembolism: the WARFASA and ASPIRE studies

The treatment of unprovoked venous thromboembolism (VTE) is anticoagulant therapy for at least 3 months [1]. As VTE recurs frequently and about 20 % of the patients with an unprovoked VTE develop recurrence within the first 2 years [2], extended vitamin K antagonist (VKA) treatment is often considered. The decision to prolong VKA therapy after the initial treatment period is a dilemma, since longer term therapy reduces the risk of recurrent thromboembolism, but increases the risk of bleeding. The last American College of Chest Physicians (ACCP) EvidenceBased Clinical guidelines suggest a strategy to balance the benefits and risks of different durations of anticoagulant therapy; in unprovoked VTE, ACCP guidelines suggest extended anticoagulant therapy if bleeding risk is low or moderate (Grade 2B) and recommend 3 months of therapy if bleeding risk is high. When extended anticoagulant therapy is contraindicated, aspirin might be appealing. In fact the annual risk of major bleeding is only 0.1 % in patients on long-term low-dose aspirin therapy, thus lower than VKA therapy [3]. A large meta-analysis by the Antiplatelet Trialists’ Collaboration shows that antiplatelet therapy significantly reduces the risk of fatal or non-fatal pulmonary embolism (PE) by 25 % [3]. Moreover, a large trial involving patients undergoing surgery for hip fracture shows a 36 % risk reduction in VTE in the aspirin therapy group, [4] suggesting that antiplatelet therapy may be an alternative to prolonged VKA therapy. Two randomized controlled studies were conducted to evaluate the use of ASA for the prevention of venous thromboembolism recurrence: the Aspirin for the Prevention of Recurrent Venous Thromboembolism (Warfarin and Aspirin [WARFASA]) [5] and the Aspirin to Prevent Recurrent Venous Thromboembolism (ASPIRE) study [6].