Reply: sex-related variations in platelet reactivity

We thank Prof. Marco Cattaneo for the thoughtful letter regarding our recent study, the largest pharmacodynamic investigation to date examining sex-related differences in platelet reactivity among patients with and without antiplatelet therapy.¹ The letter underscores the relevance of the topic of our investigation aimed at elucidating the biological underpinnings of sex-specific responses to antiplatelet agents, an issue of growing importance in contemporary cardiovascular medicine.²

Prof. Cattaneo raises the possibility that hematocrit-related artefacts may have influenced the platelet aggregation results.³ Specifically, it is suggested that lower haematocrit levels in women than in men could alter citrate distribution, leading to higher plasma calcium concentrations and potentially contributing to the observed sex differences in platelet reactivity. It is worth noting that the mentioned haematocrit-based adjustment is not part of standard operating procedures and accordingly not routinely applied in the vast majority of contemporary studies.^4–9 This hypothesis is primarily supported by a dated study from 1980 involving 11 male and 11 female healthy volunteers with a mean age of 32 years.¹⁰ While mechanistically interesting, the study presents several limitations: the sample size is small, the population is significantly younger and healthier than typical cardiovascular cohorts, and the methodological details—including the type and concentration of platelet agonists used—are not full defined and most likely not comparable to current day protocols, including ours. The reproducibility of light transmission aggregometry is highly dependent on a number of parameters, including the choice and concentration of agonist, operator expertise, and instrument calibration.⁴ For instance, the 1980 study used a platelet count adjustment technique (reconstitution of platelet-rich plasma to 200 000/μL using autologous platelet-poor plasma) not commonly employed in more recent studies—including in our analysis.^5–9,11 Moreover, in our analysis, females under 50 years of age exhibited lower platelet reactivity compared with those over 50, providing an additional explanation for the discrepancy with the 1980 study, which included a younger population of healthy volunteers.¹ Collectively, given the substantial differences in methodology, population (mean age = 32 years vs. 64 years), and sample size (n = 22 vs. n = 5687), the two studies cannot be compared. Our large-scale dataset likely offers sufficient statistical power to detect even more modest biological differences, which are typically more pronounced in the post-menopausal female population.¹²

Another important point is that the studies cited by Prof. Cattaneo are purely laboratory-based without clinical correlates. No in vitro study alone can offer conclusive evidence in this regard, and laboratory-based speculations must be interpreted cautiously. From a clinical standpoint, the primary concern lies in determining whether the observed sex differences in platelet reactivity assessed ex vivo translate into meaningful adverse outcomes. Laboratory adjustments may potentially eliminate real biological differences between sexes and may inadvertently conceal factors that help explain observed variations in treatment response and their impact on clinical outcomes. These laboratory adjustments, in fact, can interfere with measured levels of platelet reactivity and thus should be avoided, as also recommended by Cattaneo et al.¹¹ To date, numerous ex vivo studies have shown that levels of on-treatment platelet reactivity—measured using various assays—is associated with adverse clinical outcomes, both ischaemic and haemorrhagic, particularly in patients undergoing percutaneous coronary intervention (PCI).¹³ Moreover, antiplatelet therapy guided by the use of platelet function testing has been shown to improve outcomes in patient undergoing PCI.¹⁴ Accordingly, while not recommended for routine use, platelet function testing does have a role in specific clinical scenarios, as supported by international guidelines and consensus documents.^4,15 Thus, we respectfully disagree with the statement made in his letter alluding to the lack of clinical benefit associated with monitoring antiplatelet therapy with platelet function tests.

Finally, it should be noted that our results, in larger scale, are consistent with a growing body of meticulously conducted studies that report similar sex-related differences in platelet reactivity.^7,8 Moreover, our findings are aligned with recent randomized controlled trials suggesting that women may experience different clinical effects from antiplatelet therapies compared with men.^16,17 While our study results are not definitive and require further research, it does offer a potential mechanistic explanation for the differences in sex-related outcomes in patients treated with antiplatelet therapy.

Funding

This manuscript was not funded.

Data availability

Data sharing is not applicable to this article as no new data were created or analysed in this manuscript.