A RAST test is a simple blood test performed to help identify the source of an allergy. While the name is an acronym for radioallergosorbent, the RAST test is also sometimes referred to as an allergen-specific IgE antibody test. Generally, this type of test is used when a food allergy is suspected, as evidenced by the appearance of a skin rash or eczema. It may also serve as a precursor to more specific blood tests if a chronic allergic response is indicated, such as a white blood cell differential count, eosinophil count, or basophil count. The RAST test is also the standard alternative to introducing a suspected allergen directly to the patient via a skin prick test, which may cause a serious reaction.
The mechanism behind the RAST test is fairly simple. First, the test is designed to detect the presence of immunoglobulin E (IgE), a class of antibodies produced by the immune system to bind with the invading allergen and trigger the release of the inflammatory mediator, histamine. While this is measurable in itself, quantities of allergen-specific IgE antibodies that are directed to bind to certain allergens are of more interest since this indicates which specific substance(s) the patient is most likely allergic to.
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To perform the test, the clinician or technician draws a sample of blood from the patient. Then the suspected allergen is added to the blood sample in vitro. The blood sample is then rinsed to allow only the allergen-specific IgE antibodies that have bonded to the allergen to remain. Next, a dose of radioactive anti-IgE generated from patients known to be allergic to the offending substance is added to the test sample. Finally, the level and concentration of bound allergen-specific IgE antibodies are measured.
A positive RAST test, defined by elevated IgEs, suggests that an allergy is indeed present. However, there are other conditions under which a positive result may occur. For instance, a patient that has “outgrown” an allergy may continue to test positively for the same allergen for several years. It’s also important to note that a positive result is not necessarily conclusive. In addition, while an elevated concentration of allergen-specific IgEs may indicate an allergy, it does not dictate the severity of the allergic response involved.
The RAST test was developed in the 1970s by a Swedish pharmaceutical firm. Since that time, it has been modified to a newer version known as the ImmunoCAP Specific IgE test, or CAP RAST test for short. This blood test, which is considered more accurate than its predecessor, is also the only allergen-specific IgE to receive approval in the U.S. by the Food and Drug Administration.