A platelet count of more than 450,000 microliters is considered high, according to Mayo Clinic. A normal platelet count reading falls between 150,000 and 450,000 microliters.
A high platelet count, also known as thrombocytosis, may indicate an underlying medical condition, according to Mayo Clinic. Thrombocytosis may also be a side effect of certain medications. Further testing may be required for a proper diagnosis. If a patient has a consistently high platelet count over a period of time with no evident reason why, his doctor may suggest a bone marrow test. People who have high platelet counts may be at risk for blood clots.