What is a RAST Test?

Karyn Maier

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 RAST test is used to identify the source of an allergy.
The RAST test is used to identify the source of an allergy.

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.

A RAST test requires the doctor to draw blood from the patient.
A RAST test requires the doctor to draw blood from the patient.

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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.

More blood tests may follow a RAST test if chronic allergic response is indicated.
More blood tests may follow a RAST test if chronic allergic response is indicated.

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Discussion Comments


There appears to be lots of different tests out there to check for allergies. There are tests done on skin, muscle, sweat and blood. I'm not sure which is best. Has anyone had the rast blood test and could share their experience? It would help me make a decision.


I've been hearing about this RAST allergy test often in news media lately. It's sort of become a trend with famous people. They have this test done for acne issues or for dietary purposes. I've thought about having it done to see if I'm allergic to any of the foods I'm eating, but it's done in a private clinic and I don't think I want to pay for it. It makes sense to get it if there is a health condition, but not because I'm curious. Plus, I don't want the RAST allergy test results to give me a list of foods I shouldn't have, especially because they can't tell me the severity of the allergic reaction. Or maybe I no longer have allergies for it, but it still shows up in my blood. I will only get this test if I have severe allergic reaction or some other related condition.

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