The platelet count measures the number of platelets (thrombocytes) in a litre of blood. Platelets are small anucleate cell fragments derived from megakaryocytes in the bone marrow, essential for primary haemostasis through their ability to aggregate at sites of vascular injury and form the initial platelet plug.

Low platelet count (thrombocytopenia) increases bleeding risk; high platelet count (thrombocytosis) may increase clotting risk in some conditions. Platelet count is interpreted alongside platelet morphology (blood film), MPV, and clinical context. It is reported automatically as part of every full blood count.