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The RAST Allergy Test (Radioallergosorbent Test)

written by: Damon Sacks • edited by: Tania Cowling • updated: 9/27/2010

Find out more about the RAST allergy test. This test is a blood test used to acesss what allergies you may have and is an important diagnostic tool for physicians.

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    RAST Allergy Test

    The RAST test is short for radioallergosorbent test. This test is used to measure various substances that a person may be allergic to. It differs from a skin allergy test in that it is a blood test and does not use a cutaneous (skin) based response.

    Most blood tests like the RAST that are done for "allergies" are actually testing for a substance that our body produces in response to allergens called IgE. However, just finding the presence of IgE does not prove that an individual has an allergy to a substance. The marker is nonspecific and is best understood as a piece of a puzzle that may indicate an allergy when more pieces are put together.

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    Why Is RAST Used?

    Although not considered to be as specific as a skin test, it is often used as an alternative method of identifying causal agents to allergy. In vivo (in body) tests such as the RAST may be used when a patient has an active allergic response to an agent that may also affect the skin. During such a period, skin tests may be contraindicated or hard to tolerate.

    In addition, patients having a severe allergic attack may be on antihistamines. Generally, a patient who is having a skin test would need to be taken off a medication that may be providing some relief. Under such conditions, a RAST test might be preferable as it does not require discontinuation of an antihistamine such as benadryl.

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    Advantages Over Similar Tests

    In general, skin tests, not the RAST, are more likely to be ordered by your physician. Skin testing is typically considered to be faster, cheaper, and more specific. However, a RAST allergy test can have certain advantages over skin testing:

    • It poses no risk to the patient of a serious systemic allergic reaction (anaphylaxis).
    • It is better for patients unable to discontinue antihistamines, some antidepressants, beta blockers, and ACE inhibitors.
    • It is not reliant upon skin integrity or affected by skin disease.
    • It does not require a patient to tolerate symptoms of allergic response.
    • It is often better in children under 12 months of age as their skin is often not reactive enough to allergic stimulation skin tests.
    • It is considered more reliable for some venom allergies.
    • It is more convenient for the patient as it only requires submitting a blood sample.

    When considering any choices in allergy testing, a physician and patient must weigh the above advantages and agree to a diagnostic plan. Allergy testing can often be a long and frustrating process for patients. Often, pinning down an exact causal allergen may require multiple series of tests and test modalities. Speaking openly to your physician about your expectations and goals in this process may be helpful in reaching a desired outcome.

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    References

    1. Mayo Clinic: "Allergy - Tests." Retrieved electronically from http://www.mayoclinic.com/health/allergies/DS01118/DSECTION=tests-and-diagnosis on September 21, 2010.
    2. WebMD: "Allergy Tests." Retrieved electronically from http://www.webmd.com/allergies/allergy-tests on September 21, 2010.
    3. Lab Test Online: "Allergy Blood Testing." Retrieved electronically from http://www.labtestsonline.org/understanding/analytes/allergy/test.html on September 21, 2010.