BACKGROUND: Despite oral antiplatelet treatment a certain number of patients suffer adverse events while on therapy. Whether patients demonstrating low platelet inhibition in vitro are at increased risk for adverse events is unknown. The prevalence of clopidogrel resistance ranges from 4-60% in the literature and a standardized cut off value for determining resistance, protocol for treatment and testing is lacking. Characteristics of the clopidogrel resistant patient are unknown.
OBJECTIVES: The aim of the study was to determine a comparable, clinically applicable reference cut-off value for clopidogrel resistance with the VASP method in patients with coronary artery disease (CAD). Furthermore we sought to explore the prevalence of resistance in the same population while on clopidogrel treatment, and within defined patient sub-groups.
METHODS: Vasodilator-stimulated phosphoprotein (VASP) analyses, a standardized flow cytometric assay which yields a VASP platelet reactivity index (PRI) value indicating residual platelet activity, were performed on whole blood samples from 158 patients with angiographically documented CAD on clopidogrel treatment. The cut-off value for resistance was defined by the 5%-percentile in a control group of patients with CAD (n=105) being on aspirin.
RESULTS: The cut-off value for clopidogrel resistance was determined to be VASP PRI 55. 29.7% of the patients had a VASP PRI 55 while on clopidogrel treatment. We did not find any correlation between clopidogrel resistance and age (r=0.005, p=0.952). There were also no associations with gender (p=0.596), smoking habit (p=0.523), hypertension (p=0.445) or diabetes (0.498).
CONCLUSIONS: VASP analysis could be useful for monitoring response to treatment and tailoring antithrombotic drug regimens for CAD patients. This would be of great importance if future prospective clinical studies show that clopidogrel resistant patients are at higher risk for adverse events than patients with an acceptable platelet inhibition with clopidogrel.