The diagnosis of allergic purpura is not difficult based on the history and rash characteristics, but needs to be differentiated from drug purpura, idiopathic thrombocytopenic purpura, rheumatoid arthritis, septic meningitis, appendicitis, acute abdomen, glomerulonephritis or IgA nephropathy. 1, drug purpura: drug purpura purpura is characterized by no certain site, non-symmetrical, and does not appear in batches. 2, idiopathic thrombocytopenic purpura: idiopathic thrombocytopenic purpura is scattered small dots or sheets, no tendency to fusion, not higher than the skin, asymmetric distribution and reduced platelet count. 3, rheumatoid arthritis: Patients with arthritic allergic purpura have stronger joint redness, swelling, heat, pain and wandering than rheumatoid arthritis, and the rash is mostly annular erythema or polymorphic erythema. 4, septic meningitis: the rash caused by diplococcal septicemia is similar to purpura, but the symptoms of poisoning are heavy, the white blood cells are significantly higher, and the smear at the puncture rash can be positive. 5, appendicitis: allergic purpura and appendicitis both can appear around the umbilicus and right lower abdominal pain with pressure pain, but allergic purpura abdominal muscle is not tense, the skin has purpura. 6, acute abdomen: acute abdomen often shows abdominal muscle tension, pressure pain is obvious, body temperature is elevated, and even toxic shock, and white blood cells are significantly increased. 7, glomerulonephritis or IgA nephropathy: Allergic purpura patients with obvious renal symptoms should be distinguished from glomerulonephritis and IgA nephropathy after streptococcal infection, glomerulonephritis and IgA nephropathy often do not have skin purpura.