Nelson, MDMedically reviewed by Douglas A. Nelson, MD Thrombocytosis, or a high platelet count, is diagnosed when platelet levels are greater than 450,000 platelets per microliter of blood.

A high platelet count refers to the small cells that help blood clot. Also called thrombocytosis or thrombocythemia, a high platelet count is more than 450,000 platelets per microliter of blood.