Resistance to antiplatelet drugs: clopidogrel
More recently, several studies have reported non-response after administration of clopidogrel. In these communications, diverse definitions for non-response have been used. These include absolute or relative change in platelet aggregation from baseline <10%, as well as relative reduction of platelet aggregation within the lowest quartile of the cohort analyzed. 1,2 Studies analyzing large populations treated with clopidogrel, however, did not identify a discrete subset of patients with non-response who could be clearly separated from the responders. 3-5 Rather, the observed distribution of platelet responses after treatment with clopidogrel could be adequately modeled by a single normal distribution ( Figure 8.1 ). Nevertheless, the standard deviation of this distribution was very broad, comprising the range from absent platelet responses to adenosine diphosphate (ADP) to near-complete inhibition. Thus, patients identified by the various non-responder definitions published in the literature do not represent a distinct population but rather the lower end of the continuum. In the absence of an evident pharmacological definition of non-response, it is important to arrive at a clinical definition – that is, to define the threshold of on-treatment platelet reactivity above which the risk of atherothrombotic complications increases.