A platelet count is a quick, common test that measures how many platelets are in your blood. Platelets, or thrombocytes, are tiny blood cells that bud from cells in the bone marrow (megakaryocytes). Platelets form clots when there’s damage to a blood vessel. For example, if you cut your finger, platelets mix with clotting factors (proteins in the blood). Together, they form a “glue” that stops the bleeding. There are tens of thousands of platelets in a single drop of blood.