A RAST test, or radioallergosorbent test, is a type of blood test used to test for allergies. While RAST tests are expensive, and not considered as accurate as food challenges, they are considered a very safe way to test for food allergies.
When are RAST Tests Used?:
RAST tests are 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). Because the test takes place in blood drawn from the patient, and not in the patient's skin or body, there is no risk of adverse reaction as with a prick test or a food challenge.
What to Expect at the Doctor's Office:
From the patient's side, a RAST test is no more difficult or painful than any other blood test, and will only be an uncomfortable experience if you have a particular aversion to needles. You can expect to have test results within a week.
How Accurate are RAST Tests Compared to Other Allergy 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 specific IgE concentration in a patient's blood. Neither prick tests nor RAST tests, however, are considered as accurate as double-blind placebo-controlled food challenges. Where possible, a positive RAST test will be often be confirmed with a food challenge.
Advantages and Disadvantages of RAST Tests:
The major advantages of RAST tests are that they do not run any risk of adverse reaction (as prick tests, elimination diets, and food challenges do) and that they require very little time on the patient's part. 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. The major disadvantages of RAST tests are expense, accuracy (for some allergens), and the need to be confirmed with a food challenge. RAST tests don't exist for every potential allergen.
How RAST Tests Work:
Laboratory technicians add the likely allergen, bonded to a solid polymer, to the patient's blood. While there may be a large quantity of immunoglobulin E (IgE) -- the major antibody that binds to allergens in allergic reactions -- in the blood, the type that binds to each allergen is slightly different, and is referred to as allergen-specific IgE.
If the patient is allergic to the allergen being tested for, allergen-specific IgE will attach to the allergen and other IgE (from anything else the patient is allergic to) will float freely in the blood. The blood is then "washed," leaving the allergen and any of the patient's attached allergen-specific IgE.
A serum of radioactive anti-IgE, which is derived from people who are known to be allergic to the allergen being tested for, is then added, allowing technicians to determine the concentration of allergen-specific IgE in the patient's blood.
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. 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.
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.
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.