Normal platelet count is 150,000-350,000/µL. Thrombocytopenia is defined as a platelet count <100,000/µL. Bleeding time, a measurement of platelet function, is abnormally increased if platelet count <100,000/µL; injury or surgery may provoke excess bleeding. Spontaneous bleeding is unusual unless count <20,000/µL; platelet count <10,000/µL is often associated with serious hemorrhage. Bone marrow examination shows increased number of megakaryocytes in disorders associated with accelerated platelet destruction; decreased number in disorders of platelet production. Evaluation of thrombocytopenia is shown in Fig. 64-1.