Red blood spots on the arm can be considered as insect bite dermatitis, eczema, urticaria, senile hemangioma, and allergic purpura. 1. Insect bite dermatitis: when there are fusiform papular eruptions on the arm, or red bleeding spots with obvious itching and other symptoms, it may be insect bite dermatitis caused by mosquito bites, commonly found such as mosquitoes, mites, ticks and other bites. 2. Eczema: if the patient’s arm skin and red spots, papules, blisters, vesicles and other symptoms, may be eczema. The cause of the disease is not known, may be related to genetic, immune, systemic diseases, allergens, environmental temperature and humidity changes, long-term mental stress and other factors. 3. Urticaria: If the patient has limited edematous masses on the arm elevated on the skin surface with itching, it may be urticaria. Acute urticaria is mostly related to allergies and other factors; chronic urticaria is mostly related to infections, autoimmunity, mental stress and other factors. 4. Geriatric hemangioma: If the red spots on the arms of the patients are red papules with clear boundaries and different sizes, they may be geriatric hemangioma. This disease is caused by the proliferation and expansion of skin capillaries, the specific etiology is still unclear, and most of them are thought to be related to aging or gene mutation. 5. Anaphylactic purpura: if the patient’s arm appears dark red bleeding spots, symptoms of upper respiratory tract infection before the onset of the disease, accompanied by low-grade fever, loss of appetite, malaise, etc., it may be anaphylactic purpura. The cause of the disease is not clear, and may be related to infection, immune disorders, genetics and other factors. Red blood spots on the arm may also be caused by other diseases, it is recommended that patients go to the regular hospital in time, under the guidance of the doctor, to carry out relevant examinations, a clear diagnosis and follow the doctor’s instructions to accept the treatment, so as not to delay the condition.