2. Flow cytometry assays VASP phosphorylation: clopidogrel effect VASP (vasodilator-stimulated phosphoprotein) is measured in whole blood with a commercial kit (Platelet VASP; Diagnostica Stago, Biocytex) on a FACScan flow cytometer (Becton Dickinson). The results are expressed as the platelet reactivity index (PRI, %).