RAST Blood Test: An Outmoded Way to Test for Allergies

Most Doctors Have Switched to the Newer, More Sensitive ELISA Tests

Mid Section of a Nurse Injecting an Intravenous Drip in a Patient's Arm
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A RAST test is a type of blood test used to test for allergies. These tests, which use radioactivity, have largely been abandoned by doctors in favor of newer, more accurate forms of allergy testing.

RAST stands for "radioallergosorbent." RAST tests have been a very safe way to test for food allergies, but they are also expensive and are not considered as accurate as food challenges.

Medical societies and the National Institute of Allergy and Infectious Diseases recommended in 2010 that doctors switch their blood allergy testing to what's called the enzyme-linked immunosorbent assay, or ELISA, test.

This allergy test doesn't use radioactivity, and is more sensitive than the RAST test. However, a few allergists do still use RAST testing.

When Are RAST Tests Used?

In the past, RAST tests were often used in combination with skin tests, or in situations when other tests are considered risky (e.g., when a patient has experienced a severe allergic reaction after eating a food). They're still used occasionally in these circumstances, even though in the vast majority of cases doctors order ELISA tests instead of RAST tests.

Like the newer ELISA food allergy tests, the RAST test involves an allergy reaction in the patient's blood, not in the patient herself, so there's no risk of an adverse reaction. Prick tests and food challenges do carry that risk.

Advantages and Disadvantages of RAST Tests

RAST tests are considered somewhat less sensitive than skin prick tests, although they are considered basically accurate and useful.

They are valuable because they can give information about the level of allergic reaction present in a patient's blood, and they don't take much time.

They can also be used in some circumstances where prick tests cannot — for example, when the person being tested has severe eczema or persistent hives throughout the body that could throw the test off.

Neither prick tests nor RAST tests, however, are considered as accurate as "blind" food challenges, where you don't know what food you're consuming. Where possible, a positive RAST test will be often be confirmed with a food challenge. In addition, RAST tests don't exist for every potential allergen.

What to Expect from a RAST Test

Your part of a RAST test will involve a simple blood draw, which can take place at your doctor's office or at a laboratory.

Once you've provided your blood sample, the technician will add the suspected allergen to your drawn blood and see how much of that allergen attaches to immunoglobulin E (IgE) in your blood. IgE is the part of your immune system responsible for allergic reactions, and there's a slightly different form of IgE for every allergen.

Next, the lab technician will wash the blood so that only the allergen and the allergen-specific IgE remain. Finally, the technician will add a radioactive serum to the mix, which will enable the concentration of allergen-specific IgE in the patient's blood to be measured.

The Bottom Line

Most allergists are using the more sensitive ELISA tests, rather than the older, radioactive RAST tests, to test your blood for allergies. If you're not certain what test your doctor has ordered or the reasoning behind the test ordered, ask about it.

Sources

Chinoy, Birjis, Edgar Yee & Sami L. Bahna. "Skin Testing Versus Radioallergosorbent Testing for Indoor Allergens." Clinical and Molecular Allergy April 15 2005 3(4): doi: 10.1186/1476-7961-3-4. 22 Jul 2007.

Food Allergy Research and Education. Blood Tests fact sheet. Accessed Feb. 11, 2015.

Kemp, Stephen F., and Richard F. Lockey, eds. Diagnostic Testing of Allergic Disease. New York: Marcel Dekker, Inc., 2000. pp. 12, 119, 213-15.

Virella, Gabriel, ed. Medical Immunology. 5th ed. New York: Marcel Dekker, Inc., 2001. pp. 414-16.

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