We appreciate the interest of Messori and colleagues [1] in our recent meta-analysis published in the Journal. In the article, we concluded that intensifying antiplatelet therapy on the basis of platelet reactivity testing reduced cardiovascular mortality and stent thrombosis after PCI, without increasing the risk for major bleeding complications [2]. In their letter, the authors raise concerns regarding the methodology used in the meta-analysis, adding results of a post-hoc analysis using risk difference (RD) as opposed to odds ratio (OR). First of all, it should be declared that the choice of OR as an effect estimate in our meta-analysis is entirely appropriate and standard, also supported by the Cochrane collaboration and applied in several previously published impacting meta-analyses [3–6]. The preference of risk ratio (RR) and RD over OR originates from the less straightforward interpretation of the latter due to its mathematical definition that might lead to confusions if OR is misinterpreted as a RR. However, the non-equivalence of the RR and OR does not indicate that either is wrong; both are entirely valid ways of describing an intervention effect [3]. On the other hand, since OR has many advantageous features over RR or RD in statistical modeling and outcome analysis, OR and its timedependent equivalence, hazard ratio (HR) are the most frequently used effect estimates in clinical outcome analyses [3]. Likewise, using OR or HR in a meta-analysis is appropriate and has the advantage of keeping better contact between the original study results and that of the meta-analysis. Notably, the difference between OR and RR is larger in case of analyzing frequent outcomes in a study, but is ignorable in case of rare outcomes, such as in the case of our meta-analysis. Recalculating our findings by using RR as an effect estimate provided almost identical results to the originals, without any impact on statistical significance levels (Table 1). The difference between our results and the post-hoc analysis of Messori and colleagues does not originate from the fundamental differences between OR and RR, but comes from the fact that they estimated RD-s in an analysis including studies without any outcome event. In cases when the standard error cannot be computed from the available information (such as in studies without outcomes in both arms), the software performs a correction to the outcomes andproduces weighted RD values that suggest equivalence between study arms. However, this attempt is seriously questionable, because lack of events does notmeanequivalence. If no events are observed in both groups, the studyprovides no information about the relative probabilityof the event between study arms. Therefore, in such cases, the appropriate and generally applied approach is to use OR or RR as an effect estimate, and studies without events are omitted by the analysis [3]. In addition, since the included trials enrolled highly heterogeneous patient populations regarding the underlying risk for thrombotic and bleeding events, calculation of absolute risk differences (RD) instead of differences in relative event probabilities (OR or RR) is less appropriate and meaningful. However, we acknowledge that a clinical limitation to our meta-analysis was to include a large number of small studies with rare outcomes. Therefore, as highlighted in the limitation section, the exact numerical results of our meta-analysis should be interpreted cautiously as they might be subject to overestimation of treatment effect due to the small sample sizes and rare outcomes. Indeed, we believe that pooling outcome information from all the currently available, randomized, controlled clinical trials into a meta-analysis added significant novel findings to this scientific area justifying its clinical importance. In conclusion, based on the statistical analysis used, the results of our meta-analysis are valid to support the clinical benefits of intensified antiplatelet therapy based on platelet reactivity testing and the concerns of Messori and colleagues cannot be shared.
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