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ON THE COVER: Activated platelets. These irregularly shaped blood components are vital for the normal clotting process. However, platelets also play a role in blood clot formation on arterial stents or on ruptured atherosclerotic plaques, with the result of blockage of blood flow and tissue ischemia. For some patients, antiplatelet agents must be used to prevent arterial thrombosis. During therapy, platelet function tests are used to monitor response. Newer approaches to monitoring coagulation, such as viscoelastic coagulation testing, are being introduced. Controversy exists surrounding the tests for monitoring response to antiplatelet therapy or for guiding transfusion decisions in critically ill patients. This issue of Clinical Chemistry contains a Q&A in which 4 experts discuss how they define aspirin and clopidogrel response, what platelet function tests are used at their institutions, what the role of the laboratory is in guiding antiplatelet therapy, and what tests they believe have the greatest potential to predict the risk of bleeding during invasive procedures. (See page 441.) ©David Mack/Science Photo Library. Reproduced with permission.