Clopidogrel is an antiplatelet drug, blocking the ADP-pathway of platelet activation. Currently there is no universally accepted definition of clopidogrel non- responsiveness.
The difference between pre-treatment and posttreatment ADP- induced platelet aggregation has been used as an estimate of clopidogrel non- responsiveness.
The aim of the present study was to define a cut- off point for clopidogrel non- responsiveness in a study population of patients with stable coronary heart disease (CHD), and possibly give an estimation of the prevalence of non- responders in patients with stable CHD.
The study population consists of 227 consecutively enrolled patients from the ASCET-study, all treated with ASA, of which 91 were later randomized to clopidogrel. Analysis on venous blood samples were performed using VerifyNow P2Y12 assay, which measures ADP-induced platelet aggregation. We performed descriptive statistics on the outcome.
The 5% percentile of Platelet Reaction Units (PRU) at baseline was close to 170, and we used this as our cut- off- point for non- responsiveness. The equivalent 95% percentile value in % inhibition was 24%. After one month on clopidogrel treatment, we calculated a prevalence of non- responsiveness to clopidogrel of 35% and 28% for PRU and % inhibition respectively.
We found a frequency of clopidogrel non- responsiveness (28-35%) that corresponds with current literature (5-44%). As ex vivo platelet function tests not necessarily give an accurate measurement of in vivo platelet function, clinical non- responsiveness needs to be explored in larger prospective studies.