The role of aspirin in thromboprophylaxis after total hip arthroplasty (THA) is controversial. The EPCAT (Extended Prophylaxis Comparing Low Molecular Weight Heparin to Aspirin in Total Hip Arthroplasty) non-inferiority study conducted in 12 hospitals in Canada compared extended prophylaxis with aspirin and dalteparin for prevention of symptomatic venous thromboembolism (VTE) after THA. It included 778 patients who had elective unilateral THA between 2007 and 2010. After 10 days of dalteparin prophylaxis post THA, they were randomised to 28 days of dalteparin (n= 400) or aspirin (n= 386). The outcomes assessed were symptomatic VTE and bleeding. After 8 patients had withdrawn consent, 778 patients were included in the final analysis. Five of 398 patients (1.3%) on dalteparin and 1 of 380 (0.3%) on aspirin had a VTE (absolute difference, 1.0%). Aspirin was non-inferior (p< 0.001) but not superior (p= 0.22) to dalteparin. Clinically significant bleeding occurred in 5 patients (1.3%) on dalteparin and 2 (0.5%) on aspirin. The absolute between-group difference in a composite of all VTE and clinically significant bleeding events was 1.7% (p= 0.091) in favour (but not statistically significant) of aspirin. The study was halted prematurely because of difficulty with patient recruitment.
The findings of the study are limited by several factors:
• Enrolment was terminated prematurely after an unplanned interim analysis found that the primary objective of non-inferiority of aspirin had been reached. The analysis was prompted by decreasing enrolment after a major shift in the use of anticoagulant prophylaxis in Canada after the approval of rivaroxaban.
• Because the study required a daily parenteral injection for 38 days after surgery, the availability of an effective oral agent resulted in orthopaedic surgeons and their patients becoming increasingly reluctant to participate in the trial.
• There was a change in protocol to allow patients receiving long-term aspirin for other indications into the trial (though the authors do not believe that this influenced the results, as only 39 patients were in this stratum, and no thrombotic and only 1 clinically important non-major bleeding event occurred in a patient randomised to aspirin).
Overall the generalisability of these study findings may be questioned given the recent availability of novel oral anticoagulants for VTE prophylaxis after THA; whether aspirin is as effective and safe as these newer agents for this indication requires further study.
NICE guidance on dabigatran and rivaroxaban supports their use as an option for the prevention of VTE in adults undergoing elective total hip or total knee replacement surgery.