Purple blood spots on the feet of the elderly may be due to allergic purpura, thrombocytopenic purpura and so on. 1.allergic purpura: immune complex-mediated leukocyte-crushing vasculitis, may be related to infection, vaccine, food contact allergens, etc., occurring in both lower limbs and buttocks, manifested as a pinpoint to soybean-sized dark red petechiae ecchymosis, may be accompanied by arthralgia, abdominal pain, hematuria, proteinuria and so on. 2. Thrombocytopenic purpura: caused by platelet generation disorder or ineffective generation, increased platelet destruction or excessive consumption, manifested as skin and mucous membrane petechiae, purpura, ecchymosis, may also appear nosebleed, oral blisters and other clinical manifestations. For the elderly feet appear purple blood spots, there may be other reasons, need to go to the hospital for examination, under the guidance of the doctor for the specific cause of treatment.