MODEL RELEASED. RAST blood test. Doctor taking blood from the arm of a 10-year-old boy to perform an allergy screen. A nurse is talking to the boy to distract him. The blood will be sent to a lab to undergo a radioallergosorbent (RAST) test, which is used to identify if a patient has a 'true' allergy. The blood is mixed with the suspected allergen and then checked for IgE antibodies. If they are present the patient has an allergy. IgE is produced by white blood cells. It interacts with the allergen-causing chemicals, for example histamine, to be produced. Such chemicals cause the symptoms of an allergy.