Generally, a count that is over 500,000 platelets per microliter of blood is considered high enough to seek the underlying cause, notes Mayo Clinic. A normal platelet range is 150,000 to 450,000 platelets. A high platelet count is referred to as thrombocytosis and has many possible causes.
A patient who has a high platelet count may have reactive or essential thrombocytosis, states Mayo Clinic. Reactive thrombocytosis is caused by a wide variety of conditions, such as allergic reactions, cancer or major surgery. Some medications also cause a high platelet count. Essential thrombocytosis occurs because the body overproduces the cells that form platelets. This is a bone marrow disorder.