The PFA-100 is an in vitro diagnostic test of primary hemostasis, the formation of a primary platelet plug. The test is performed by forcing a blood sample through a hole in a membrane coated with collagen and either ADP or epinephrine. The endpoint of the test is the time required for the platelet plug to form and stop flow through the hole. The test is prolonged in patients with disorders of primary hemostasis (e.g. inherited and acquired platelet function defects, von Willebrand disease). The test is not specific; further investigation is required to define the cause of a prolonged closure time, for example, platelet aggregometry in cases of suspected primary platelet dysfunction or assessment of von Willebrand factor in suspected von Willebrand disease. Patients with a hematocrit <35% or platelet count <150,000 may have prolonged closure times. This test may not detect subtle platelet defects; therefore if the PFA is normal, but the clinical suspicion of a platelet disorder is high, then platelet aggregometry is recommended.