A high platelet count is when the number of platelets in the peripheral blood exceeds the normal range. In clinical practice, there are two types of excessive platelets: the first condition is called reactive platelet elevation, which is an increase in platelet count due to external factors. Common triggers include major surgery, trauma, bleeding, and infection, such as craniotomy, which is a major surgery. Some patients will then have a significantly higher reactive platelet count. The second cause of long platelet increase is myeloproliferative disorders, which commonly include primary thrombocytosis, true erythrocytosis, and some patients with chronic granulocytic leukemia, all of whom will have abnormally high platelet counts. In particular, in primary thrombocythemia, abnormally high platelet counts can occur. In patients with chronic granulocytic leukemia, a persistently high platelet count often represents a progressive disease with a poor prognosis.