Platelets, also called thrombocytes, are small disk-shaped cytoplasmic fragments produced in the bone marrow and involved in the process of blood clotting. Platelets are not true cells because they do not contain nuclei. When a blood vessel is injured, platelets first adhere to the site of the injury. Then, during the activation phase of clotting, the platelets bind to each other. Platelets react with a protein in the blood called fibrinogen to form fibrin, a thread-like substance that helps create a web-like structure of blood cells and platelets at the injury site, ultimately forming a clot to stop the bleeding. There are normally between 150,000-450,000 platelets in each microliter of blood. Low platelet counts or abnormally shaped platelets are associated with bleeding disorders. High platelet counts sometimes indicate disorders of the bone marrow.

A platelet count is a diagnostic test that determines the number of platelets in the patient's blood. The primary functions of a platelet count are to assist in the diagnosis of bleeding disorders and to monitor patients who are being treated for any disease involving bone marrow failure. Patients who have leukemia, polycythemia vera, or aplastic anemia are given periodic platelet count tests to monitor their health.

An abnormally low platelet level (thrombocytopenia) is a condition that may result from increased destruction of platelets, decreased production, or increased usage of platelets. In idiopathic thrombocytopenic purpura (ITP), platelets are destroyed at abnormally high rates. Hypersplenism is characterized by the collection (sequestration) of platelets in the spleen. Disseminated intravascular coagulation (DIC) is a condition in which blood clots occur within blood vessels in a number of tissues. All of these diseases produce reduced platelet counts.

Abnormally high platelet levels (thrombocytosis) may indicate either a benign reaction to an infection, surgery, or certain medications; or a disease like polycythemia vera, in which the bone marrow produces too many platelets too quickly.