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Aspirin is a modest inhibitor of platelet function, and platelets participate in the initiation of venous thrombosis, but whether aspirin is effective in preventing venous thromboembolism (VTE) is unclear. In the Pulmonary Embolism Protection study, researchers showed that aspirin decreased VTE incidence among patients who had experienced hip fracture or undergone elective hip arthroplasty (Lancet 2000; 355:1295). Aspirin also lowered risk for VTE in patients with multiple myeloma who were receiving chemotherapy with dexamethasone and thalidomide (Mayo Clin Proc 2005; 80:1568). However, aspirin did not lower the incidence of VTE among air travelers (J Gen Intern Med 2007; 22:107) and was less effective than other VTE prophylactic agents in high-risk orthopedic patients (Chest 2004; 126:338S).
To examine whether long-term use of low-dose aspirin lowers risk for VTE in middle-aged women (age, ≥45), Harvard University investigators performed a subgroup analysis of nearly 40,000 women who were enrolled in the Women’s Health Study. During a 10-year observation period, women took either aspirin (100 mg every other day) or matched placebo. Every 6 months, the participants completed questionnaires about adherence to aspirin therapy, adverse effects, risk factors, and whether they had experienced VTE episodes. New VTE episodes were confirmed by a review of medical records for objective evidence of deep venous thrombosis (DVT) or pulmonary embolism (PE).
Women in the aspirin group reported 235 VTE episodes, and women in the placebo group reported 247, which was a nonsignificant difference. Similar numbers of DVTs and PEs occurred in each group. Although a slight benefit from aspirin was observed in older women (age ≥65; relative hazard, 0.67; P=0.055), no differences were observed in other analyzed subgroups that were designated by body-mass index, hormonal therapy use, or VTE history. Incidence of VTE was 2.7 times higher in women with thrombophilic mutations, but no differences were found in the rate of VTE between aspirin and placebo recipients in this subgroup. Overall, hemorrhagic stroke incidence was higher in the aspirin group than in the placebo group (relative hazard, 1.24).
Glynn RJ et al. Effect of low-dose aspirin on the occurrence of venous thromboembolism: A randomized trial. Ann Intern Med 2007 Oct 16; 147:525.
Comment
Results of this randomized, double-blind study showed no difference in the incidence of VTE between the placebo group and the aspirin group, in which nearly 20,000 women took 100 mg every other day for 10 years. In contrast, aspirin use was associated with a small but substantial risk for intracranial hemorrhage. The current evidence suggests that aspirin is ineffective for preventing VTE in people at low or moderate risk, although it might have some efficacy in people at high risk (e.g., after orthopedic injuries or surgery or in association with cancer chemotherapy). However, in these high-risk patients, low-molecular-weight heparin likely is a more-effective prophylactic agent than aspirin.