The Impact [cone and plate(let) analyzer (CPA) Technology]is testing whole blood platelet adhesion and aggregation under arterial flow conditions. The adhered platelets are stained and the percentage of surface covered (SC) by the stained objects and the average size (AS) of the objects are determined by image analyzer. The system allows screening of congenital primary hemostasis abnormalities, including adhesion as well as aggregation defects. The observation that exposure of blood to suboptimal concentrations of platelet agonists for 1 min prior to the Impact test causes platelet adhesion refractoriness led to the development of Impact aggregation tests and of specific tests for the determination of platelet response to aspirin and clopidogrel.