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Blood Tests for Food Allergies

written by: CoDayDreamer • edited by: lrohner • updated: 12/24/2010

Skin tests and blood tests are used for determining allergies. RAST and ELISA are types of allergy blood tests. There are times when a skin test is not appropriate. Learn more about when a food allergy blood test is necessary and what the results mean.

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    Overview

    Allergy testing can be completed by radioallergosorbent testing (RAST) and enzyme-linked immunoassay testing (ELISA). RAST and ELISA are used to determine food allergies. There are conditions which require a food allergy blood test instead of a skin test.

    During the skin testing procedure, a small amount of suspected allergen is placed under the skin. If there is a reaction, such as raised area, redness or itching, an allergy to the substance may be indicated. The RAST and ELISA tests require a small sample of blood to be taken. The sample is then sent to a laboratory for review.

    A physician will determine the best testing method based on the possible allergen, medications, age of the individual and skin sensitivities.

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    Indications

    An individual may need to have blood testing for an allergy under the following conditions:

    • Age - Individuals that are young may not be able to stay motionless for the time required to complete skin testing. Skin testing typically requires many substances to be introduced to the skin through many needles. RAST and ELISA allow the physician to take blood using one needle.
    • Severity - If an individual is suspected of having a severe allergic reaction, it is safer to take blood and test it rather than introduce the allergen into the individual's skin.
    • Skin condition - Individual's with skin conditions, such as hives, eczema and psoriasis, may exacerbate their skin condition with skin tests. The skin test result may be difficult to determine in patients with skin conditions.
    • Medication requirement - Antihistamines cannot be taken prior to skin testing. When antihistamines are taken prior to testing, the skin test result may not be accurate. Antihistamines prevent allergic reactions from occurring to the skin. Blood tests can be completed when antihistamines are present.
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    Determination

    RAST is completed by taking a small amount of blood and examining the IgE antibodies. IgE is a type of immunoglobulin. If an IgE test is positive, it indicates that an individual is probably allergic to the substance. Likewise, if an IgE test is negative, it indicates that the individual is probably not allergic.

    ELISA blood samples are analyzed for enzymes. Each potential allergen has an antibody which in turn has a corresponding enzyme. The testing procedure verifies if these enzymes are present.

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    Testing Limitations

    The blood test can only predict the likelihood that an individual is or is not allergic to a substance. They cannot determine the severity of an allergic reaction. If the blood test indicates that an allergy is likely, it is possible that an individual may not have a reaction when exposed.

    When skin testing is completed, the results are available within minutes. Blood testing results are not immediate. Once the blood sample is obtained, the blood is typically sent to a laboratory for review. The laboratory will review the blood based on the physician's request. When the review is complete, the results are then sent back to the physician whom will notify the individual.

    Individuals should contact their physician if they are concerned about an allergy. A physician will be able to review the individual's complete medical history, determine if a food allergy blood test is necessary and properly diagnosis the allergy.

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    References

    Lab Tests Online: Allergy Blood Testing: The Test: http://www.labtestsonline.org/understanding/analytes/allergy/test.html

    WebMD: Blood Testing for Allergies: http://www.webmd.com/allergies/blood-testing

    American College of Allergy, Asthma & Immunology: Allergy Tests: About Allergy Testing: http://www.acaai.org/allergist/allergies/Treatment/diagnosing-allergies/Pages/allergy-testing.aspx

    MedlinePlus: ELISA: http://www.nlm.nih.gov/medlineplus/ency/article/003332.htm