Platelets are small, cell-like particles produced in the bone marrow. Their main function is to start the formation of blood clots. Platelets gather where bleeding occurs and clump together to form the initial plug that stops or slows down the flow of blood. Platelets also release other substances needed to complete the clotting process.
Platelet disorders can result from having too few or too many platelets or from impaired platelet function. In general, when platelet counts fall very low there is an increased risk of bleeding. Decreased numbers of platelets may be caused by drugs, toxins, or disorders of the bone marrow. Conditions that consume a large number of platelets (such as massive bleeding or severe clotting disorders) can also deplete platelet numbers. Finally, large numbers of platelets can become trapped in an enlarged spleen, decreasing the number of platelets in the blood.
An abnormal increase in the number of platelets is rare and often the cause is not known. It may be associated with bone marrow disease or with longterm blood loss and iron deficiency. There are also disorders in which platelets do not function properly.