A platelet count is a blood test used to measure the amount of platelets in the body, according to MedlinePlus. Platelets are the part of the blood responsible for clotting, and their number can signal different types of diseases, such as cancer.
The blood for the test is usually drawn from a vein, according to MedlinePlus. The site is cleaned, and a needle is inserted, usually in the elbow or the back of the hand. Blood is then collected and tested.
Normal results range between 150,000 and 400,000 platelets per microliter, notes MedlinePlus. A low platelet count may indicate an autoimmune disorder. A high platelet count may indicate the presence of certain kinds of cancer and may increase a person's risk of developing blood clots.