Radioallergosorbent test: An allergy test done on a sample of blood. The test is used to check for allergic sensitivity to specific substances. In the test, the sample of blood is mixed with substances known to trigger allergies. The test measures the level of allergy antibodies (specific IgE antibodies) in the blood which are present if there is a allergic reaction. Because Radioallergosorbent test is a mouthful, it is best known as RAST.
The advisory panel on allergy, Internal medicine and otolaryngology believed that radioallergosorbent test (RAST) is a recognised and effective test of allergy.
RAST detect specific IgE antibodies responsible for hypersensitivity: the allergen is bound to insoluble material and the patient's serum is reacted with this conjugate; if the serum contains antibody to the allergen, it will be complexed to the allergen. Radiolabeled anti-human IgE antibody is added where it reacts with the bound IgE. The amount of radioactivity is proportional to the serum IgE.
RAST tests are often used in combination with skin tests, or in situations when other tests are considered risky (e.g., when a patient has experienced a severe allergic reaction after eating a food). Because the test takes place in blood drawn from the patient, and not in the patient's skin or body, there is no risk of adverse reaction as with a prick test or a food challenge.
Laboratory technicians add the likely allergen, bonded to a solid polymer, to the patient's blood. While there may be a large quantity of immunoglobulin E (IgE) -- the major antibody that binds to allergens in allergic reactions -- in the blood, the type that binds to each allergen is slightly different, and is referred to as allergen-specific IgE.
If the patient is allergic to the allergen being tested for, allergen-specific IgE will attach to the allergen and other IgE (from anything else the patient is allergic to) will float freely in the blood. The blood is then "washed," leaving the allergen and any of the patient's attached allergen-specific IgE.
A serum of radioactive anti-IgE, which is derived from people who are known to be allergic to the allergen being tested for, is then added, allowing technicians to determine the concentration of allergen-specific IgE in the patient's blood.