What could cause high platelet count in blood?

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.