Clopidogrel has a known biological variability that has been consistently associated with recurrence of coronary ischemic events in clinical studies. Among the tests that are currently available, quantification of the phosphorylation status of the vasodilator phosphoprotein (VASP assay) is probably the most specific assay to evaluate the inhibition of the P2Y12 receptor by clopidogrel. A genetic polymorphism of the cytochrome 2C19 has been associated with the biological efficacy of clopidogrel and is also associated with recurrent ischemic events. The VASP assay and the 2C19 genotyping are candidates for the identification of patients at risk; this is the focus of the present review.