The RAST test is a blood test used to identify the source of an allergy by detecting allergen-specific IgE antibodies. It is commonly used for suspected food allergies and is a safer alternative to skin tests. A positive result suggests an allergy is present, but it does not determine the severity of the allergic response. The RAST test has been updated to the ImmunoCAP Specific IgE test, which is considered more accurate and has FDA approval in the US.
A RAST test is a simple blood test done 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. Typically, this type of test is used when a food allergy is suspected, as evidenced by the appearance of a rash or eczema. It can also serve as a precursor to more specific blood tests if a chronic allergic response is indicated, such as a differential white blood cell 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 test, which can cause a severe reaction.
The mechanism behind RAST testing is quite 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 to the invading allergen and trigger the release of the inflammatory mediator, histamine. While this is measurable in itself, the amounts of allergen-specific IgE antibodies that are directed to bind to certain allergens are of greater interest as this indicates which specific substance the patient is most likely to be allergic to.
To perform the test, the doctor or technician takes a blood sample from the patient. Then the suspected allergen is added to the in vitro blood sample. The blood sample is then rinsed to allow only the allergen-specific IgE antibodies that have bound to the allergen to remain. Next, a dose of radioactive anti-IgE generated by patients known to be allergic to the offending substance is added to the test sample. Finally, the level and concentration of the bound allergen-specific IgE antibodies are measured.
A positive RAST test, defined by elevated IgE, suggests that an allergy is indeed present. However, there are other conditions under which a positive result can occur. For example, a patient who has “outgrown” an allergy may continue to test positive for the same allergen for several years. It is also important to note that a positive result is not necessarily conclusive. Furthermore, while an elevated concentration of allergen-specific IgE may indicate an allergy, it does not determine the severity of the allergic response involved.
The RAST test was developed in the 1970s by a Swedish pharmaceutical company. Since that time, it has been changed to a newer version known as the ImmunoCAP Specific IgE test or CAP RAST test for short. This blood test, considered more accurate than its predecessor, is also the only allergen-specific IgE to receive approval in the United States from the Food and Drug Administration.
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