Rare, chronic myeloproliferative neoplasm
Abnormal proliferation is seen in megakaryocytic line only
Typically in elderly
Generally asymptomatic
Essential thrombocytosis
Myelofibrosis
Polycythemia vera
Chronic myelogenous leukemia 
Immune thrombocytopenic purpura
Polycythemia vera 
Thrombotic thrombocytopenic purpura
Acute blood loss