Particles in blood that help stop bleeding if you cut or injury yourself. A low platelet count (called thrombocytopenia) can cause spontaneous bleeding (haemorrhage) and bruising. A high platelet count (called thrombocytosis) can cause thrombosis (excessive clotting of blood restricting blood flow). Extremely elevated platelets can cause haemorrhage. Your doctor usually measures your platelet count on routine blood tests.