A radioallergosorbent test, also commonly known as an RAST test, is one of the foremost medical methods of determining the presence of a food allergy in an individual. This is accomplished primarily by measuring the amount of IgE antibody within the individual's bloodstream against a specific allergen. Another commonly used name for an RAST blood test is an allergen specific IgE blood test. The test is also generally used as a precursor to more specific blood tests that show up with chronic allergic responses such as white blood cell differential count or basophil count. This is a much preferred option as compared to the age old method of introducing a suspected allergen directly to the patient via a skin prick test, something that is known to have dire consequences in serious cases. The test is primarily recommended when the patient experiences an outbreak of severe dermatitis or eczema, or even if a dangerous allergic reaction is expected to follow another test. This test is also commonly performed in order to measure the immunotherapy of a child in order to substantiate if the individual has outgrown an allergy. It is important to note that the level of IgE does not in any way directly correlate to the severity of an allergic reaction. People that have outgrown a specific allergy are known to find positive IgE's show up for them many years later.
A RAST blood test is fairly simple and will first attempt to detect the presence of immunoglobin E (or IgE as stated above). This is a class of antibodies that are produced by the immune system and tend to bind themselves to the invading allergen before triggering the release of histamine, which is an inflammatory mediator. The test is performed with the medical staff or technician drawing a blood sample from a vein in the patients arm and treating that blood with the suspected allergen. The RAST test results will indicate the presence of an allergy if declared positive and is defined by the elevated IgE's. However, this is by no means a definitive medical test as, as mentioned previously, there is a chance an individual will test positive for elevated IgE's even if he or she has outgrown the allergy. In most cases, addition skin testing as well as office challenges may be required to clarify the significance and accuracy of a RAST test