Aspirin in primary cardiovascular prevention: the two faces of the coin and the importance of the Number Needed to Treat: a systematic review and meta-analysis.

Aspirin has been an essential treatment for the primary prevention of cardiovascular diseases (CVD). Several randomized controlled studies do not support the routine use of aspirin, mainly due to its association with bleeding risk. This systematic review aims to advocate aspirin prescription based on the Number Needed to Treat (NNT) and the Number Needed to Harm (NNH). This combination provides a good measure of the effort to avoid an unfavorable outcome, weighed against possible associated risks. A search of randomized studies on aspirin treatment was conducted in two separate periods. Four studies from 1988-1998 and six from 2001-2018 were included in the analysis (157,060 participants). The primary endpoint was a composite outcome of Nonfatal Myocardial Infarction (NFMI), Non-fatal Ischemic Stroke (NFIS), and CV mortality. Major bleeding was a safety endpoint. We calculated the Absolute Risk Reduction (ARR%), NNT, and NNH, alongside the Relative Risk (RR) and 95% CI of each primary endpoint. The results of all included studies (10) showed a net benefit with aspirin treatment for NFMI (NNT= 259) and the composite outcome (NNT=292) with a significant relative risk reduction of 20% (p=0.003; I-2= 0%) and 10% (p<0.001; I-2= 0%), respectively. There was a relevant 60% increase in the bleeding risk (p<0.0001, NNH=208; I-2= 3%). The NNT and NNH may constitute measures of efficacy and risk in clinical shared decision-making. However, it is essential to consistently establish that patients' benefit-risk should be individualized and not represent a clinical guide for everyone.