Purple spots on the legs are petechiae or petechiae, which are caused by bleeding from broken capillaries under the skin. The most common type is allergic purpura or thrombocytopenic purpura, and both can be identified by routine blood tests. If the platelet count is normal, it is allergic purpura. Thrombocytopenic purpura will be accompanied by a reduced platelet count and a generalized bump will cause petechiae or ecchymosis. Allergic purpura is a hypersensitive reactive inflammatory disease of capillaries and small blood vessels, divided into simple, arthritic, abdominal and renal types. Further examination of blood and urine routine is needed to determine whether there is infection as well as kidney damage, and appropriate medication should be selected according to the corresponding symptoms.