If purple blood dots on the body occur by chance and the area of the dots is small and the number of dots is small, this condition may be caused by accidental impact or squeeze causing capillary rupture and bleeding, generally no further examination is needed. However, if the purple blood dots are larger and more dense, this condition is highly suspected purpura. Purpura is divided into two conditions: one is allergic purpura; one is thrombocytopenic purpura, both of which are the cause of purple blood dots, but their pathogenesis is different. Allergic purpura is due to allergic reaction caused by taking certain drugs or food, which leads to increased permeability of blood vessels, so small blood vessel rupture will occur causing bleeding. In contrast, thrombocytopenic purpura is caused by a decrease in the number of platelets and this bleeding is usually accompanied by nosebleeds, gum bleeds or blood in urine or stool.