A radioallergosorbent test (RAST) is a blood test using radioimmunoassay test to detect specific IgE antibodies, to determine the substances a subject is allergic to. This is different from a skin allergy test, which determines allergy by the reaction of a person's skin to different substances. A radioallergosorbent test (RAST) is a blood test using radioimmunoassay test to detect specific IgE antibodies, to determine the substances a subject is allergic to. This is different from a skin allergy test, which determines allergy by the reaction of a person's skin to different substances.