A normal platelet count ranges from 150,000 to 400,000 platelets per microliter, according to MedlinePlus. Because normal values vary slightly depending on the lab, it is important to discuss the results with a doctor.
Smaller than red or white blood cells, platelets are a part of the blood that assist in clotting, states MedlinePlus. Common causes of low platelet counts include chemotherapy, radiation and autoimmune disorders. Certain medications may also lead to a low platelet count.
Conditions such as cancer and certain types of anemia sometimes lead to a high platelet count. Recent spleen removal, major surgery, trauma, allergic reactions and certain infections may also cause elevated platelet counts.