Platelet factor 4

Platelets, also known as thrombocytes, are small blood cells. They form in your bone marrow, the sponge-like tissue in your bones. Platelets are essential for blood clotting. A blood clot is a mass of blood that forms when platelets, proteins, and cells in the blood stick together. When you get hurt, your body forms a blood clot to stop the bleeding.

A platelet count is most often used to monitor or diagnose conditions that cause too much bleeding or clotting. A platelet count may be included in a complete blood count, a test that is often done as part of a regular checkup.