Platelet count >350,000/µL. Either primary (essential thrombocytosis; Chap. 65. Myeloid Leukemias, Myelodysplasia, and Myeloproliferative Syndromes) or secondary (reactive); latter secondary to severe hemorrhage, iron deficiency, surgery, after splenectomy (transient), malignant neoplasms (esp. Hodgkin's lymphoma, polycythemia vera), chronic inflammatory diseases (e.g., inflammatory bowel disease), recovery from acute infection, vitamin B₁₂ deficiency, drugs (e.g., vincristine, epinephrine). Rebound thrombocytosis may occur after marrow recovery from cytotoxic agents, alcohol, vitamin B₁₂ replenishment. Primary thrombocytosis may be complicated by bleeding and/or thrombosis; secondary rarely causes hemostatic problems.