A platelet count is a test that measures the number of platelets in a persons blood. Platelets are cells that help the blood to clot, and they are produced in the bone marrow. A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood. Having more than 450,000 platelets is a condition called thrombocytosis, while having less than 150,000 is known as thrombocytopenia. A low platelet count can make it difficult for the blood to clot, putting a person at risk of excessive bleeding. Low platelet counts can be due to an inherited medical condition, an acquired medical condition, or unknown causes. A high platelet count can occur when something causes the bone marrow to make too many platelets, and it can put a person at risk of blood clots or stroke. If a persons platelet count falls below 10,000 platelets per microliter, it can lead to dangerous internal bleeding.