What causes a high blood platelet count?

Thrombocytosis...: ...Is the clinical term for a platelet count > 450k. This is subclassified as primary (essential) vs. Secondary: the former is due to unexplained production of platelets, the latter is most often due to chronic infections or inflammation (crohn's dz), anemia, cancer, a rxn to a medication, or following the surgical removal of the spleen. For more info check www.Thrombocytosis.Net.

Many causes: Assuming no recent injury / surgery. The most common cause of a slightly elvated platelet count (less than double the reference range top) is iron deficiency which may not yet be producing anemia. A very high platelet count that stays way-high is often due to a low-grade proliferative disease of platelets that a hematologist can usually manage.