Platelets, also called thrombocytes, are tiny fragments of cells that are essential for normal blood clotting. They are formed from very large cells called megakaryocytes in the bone marrow and are released into the blood to circulate. The platelet count is a test that determines the number of platelets in a person's sample of blood.When there is an injury to a blood vessel or tissue and bleeding begins, platelets help stop bleeding in three ways. They:

Adhere to the injury site,
Clump together (aggregate) with other platelets,
Release chemical compounds that stimulate further aggregation of other platelets,These steps result in the formation of a loose platelet plug at the site of the injury in a process called primary hemostasis. At the same time, activated platelets support the coagulation cascade, a series of steps that involves the sequential activation of proteins called clotting factors. This secondary hemostasis process results in the formation of strands of fibrin that weave through the loose platelet plug, form a fibrin net, and compress to form a stable clot that remains in place until the injury has healed. When the clot is no longer needed, other factors break the clot down and remove it.Each component of primary and secondary hemostasis must be present, activated at the right time, and functioning properly for adequate clotting. If there are insufficient platelets, or if platelets are not functioning normally, a stable clot may not form and a person may be at an increased risk of excessive bleeding.Platelets survive in the circulation about 8 to 10 days, and the bone marrow must continually produce new platelets to replace those that degrade, are used up, and/or are lost through bleeding. Determining the number of platelets in blood with a platelet count can help diagnose a range of disorders having to do with too few or too many platelets.