The clinical characteristic of allergic purpura is a rash, typically characterized by a high plane and symmetry of both lower extremities, which can be seen by an experienced doctor at a glance. However, special care should be taken for less typical rashes or those with irregular onset. Some rashes are not necessarily on both lower extremities, such as between the hair, behind the ears, on the face, on the upper extremities or on the trunk. This atypical situation often makes it easy to misdiagnose or miss the diagnosis. Therefore, parents should always seek a clear diagnosis from a pediatrician once they find a rash in their child; if a general pediatrician has difficulty diagnosing it, they should always seek a specialist for identification. The diagnostic criteria of allergic purpura include routine blood tests, blood sedimentation, serum immunoglobulin, urine routine, fecal occult blood, etc. The main point of diagnosis is to have a history of infection, food, drug or pollen allergy, insect bite, vaccination, etc. Diagnostic criteria of allergic purpura 1, blood routine examination: blood cells are mild to moderately increased eosinophils are normal or increased, bleeding can be more anemic, clotting time, platelet count, clot contraction time are normal. 2.Blood sedimentation: most patients have increased blood sedimentation. 3.Anti-O: may be increased. 4.Serum immunoglobulin: serum LGA may be increased. 5.Urinary routine: protein red blood cells or tubular type may appear in the urine of those with kidney involvement. 6.Blood urea nitrogen and kegan: increased in renal insufficiency. 7.Fecal occult blood: positive type in case of gastrointestinal bleeding. 8.Capillary fragility test: positive in about half of the patients. 9, kidney tissue biopsy: can determine the nature of nephritis lesion has a guiding significance for the determination of treatment and prognosis. Allergic purpura diagnosis points 1, most have a history of infection, food, drug or pollen allergy, insect bite, vaccination, etc. 2.There are typical characteristic skin purpura combined with joint, gastrointestinal or kidney symptoms and history of recurrent attacks. 3, Increased leukocytes and eosinophils in whole blood, and decreased erythrocytes and hemoglobin when bleeding is severe. 4.Blood sedimentation is increased, CPR may be positive, and serum IgA is increased. 5.When there is renal damage, hematuria and proteinuria can be seen.