PRINCIPLE OF THE TEST SYSTEM The instrument aspirates a blood sample under constant vacuum from the sample reservoir through a capillary and a microscopic aperture (147 μm) cut into the membrane. The membrane is coated with CEPI or CADP. The presence of these platelet activators and the high shear rates (5000 to 6000 s–1) under the standardized flow conditions result in platelet attachment, activation, and aggregation, building a stable platelet plug at the aperture.5 The time required to occlude the aperture is reported as CT, and measurements are finished after a maximum of 5 minutes. In vitro experiments have demonstrated that antibodies against GPIb, GPIIb/IIIa, vWF, and RGDS peptide—but not against fibrinogen—induce a dose-dependent prolongation of CTs.6 This study indicated that the primary adhesion process occurs through vWF-GPIb interaction and that platelet plug formation involves the interaction of vWF with GPIIb/IIIa. These findings were confirmed and extended by another study with various antibodies against vWF or GPIb.7 Two types of cartridges have been developed to differentiate between the aspirin-induced defect (which can be detected with the CEPI cartridge) and more severe platelet dysfunction (detected with the CADP cartridge). In clinical practice the CEPI cartridge is used first: when CEPI is abnormal, a second measurement is made with the CADP cartridge.4,8 Normal values of CT are provided in Table I. Most of the cited artiREVIEW ARTICLES