A blood test for allergy evaluation, commonly called a RAST test (short for radioallergosorbent test) may be used to determine to what substances a person is allergic. The RAST test, which is more correctly termed an in vitro radioimmunoassay, works by measuring in the serum whether or not person has a significant amount of Immunoglobulin E (IgE), the antibody that is elevated in allergic individuals. This is different from a skin allergy test, which determines allergy by measuring the amount of reaction of a person's skin to different allergens when these are injected into the outer layer of the skin.
In general, this method of blood testing (in-vitro, out of body) vs. skin testing (in-vivo, in body) is most useful for the following patients: Those who cannot stop taking an antihistamine or other medicine that could affect skin test results; a pediatric patient, or in the presence of a skin condition such as severe eczema or hives where skin testing may be impossible to interpret. In some uncommon cases a patient has such a high sensitivity level to suspected allergens that any injected administration of those allergens for skin testing might result in potentially serious side effects. Still, under most circumstances, allergy skin testing is our preferred testing method in comparison with various in vitro tests for assessing the presence of specific IgE antibodies because it is more sensitive, the results can be obtained within minutes, and it is less expensive.
The RAST test is scored on a scale from 0 to 5 (0= absent or very minimal allergen specific IgE; 5 =extremely high level of allergen specific IgE).