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We appreciate and share the interest of Dr. Varinasi and colleagues in aspirin resistance and its clinical relevance. Previously, we documented the profile and prevalence of aspirin resistance in stable patients with cardiovascular disease (1). In this initial study, we used both optical platelet aggregation, which we consider to be the gold standard for the determination of platelet reactivity in the presence of aspirin, and a rapid, whole-blood assay, the platelet function analyzer (PFA)-100, to determine the prevalence of aspirin resistance. The kappa statistic between these two methods was 0.1 (95% confidence interval 0.045 to 0.246), indicating a poor correlation between optical platelet aggregation and the PFA-100 in detection of aspirin resistance.
In our more recently published work (2), we reported an increased risk of death, myocardial infarction (MI), or stroke associated with aspirin resistance as determined by optical platelet aggregation. In analysis, long-term outcomes (death/MI/stroke) were not related to aspirin resistance status as determined by the PFA-100 (12.9% aspirin sensitive vs. 15.1% aspirin resistant, p = 0.4). These findings seem to indicate that the PFA-100 is not as specific a test as compared to optical platelet aggregation for determining clinically relevant aspirin resistance. In fact, this supposition may be supported by the poor kappa statistic between the two tests. However, prior to categorically drawing this conclusion, one must acknowledge the real possibility of a type II error. Although there may be no statistical association between the PFA-100 and clinical outcomes in our investigation, a real association may have been missed by the small sample size of our study.
As reported, our study did demonstrate aspirin resistance as measured by optical platelet aggregation to be independently predictive of poor long-term outcomes, but further work is needed in this area. Larger studies evaluating more widely available point-of-care tests are needed to define a test that has both ease of use and clinical relevance.
- American College of Cardiology Foundation