Platelet function testing in the clinical setting is usually performed in a specialized laboratory where sufficient numbers of patients can be studied on an annual basis to ensure reproducibility and correct interpretation of the results. Although there are clearly exceptions, platelet function tests are usually performed either to determine the cause of bleeding in a nonthrombocytopenic patient with normal coagulation parameters or to assess the impact of antiplatelet agents, such as aspirin, the P2Y₁₂ ADP receptor antagonists. With the exception of von Willebrand's disease, most hereditary platelet disorders are rare. Leaving aside patients receiving drugs that affect platelet function, a well-characterized mechanism is identified less than half the time.