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Allergy testing

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RAST test Allergy skin prick or scratch test Intradermal allergy test reactions Skin testing, PPD (R arm) and Candida (L)

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Allergy testing

Definition:

Allergy tests are any of several tests used to determine the substances to which a person is allergic.

Alternative Names:
Patch tests - allergy; Scratch tests - allergy; Skin tests - allergy; RAST test

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How the test is performed:

There are many methods of allergy testing. Among the more common are the skin tests, elimination-type tests, and the radioallergosorbent test (RAST).

Skin tests are the most common. Specific methods vary. The scratch test (one of the most common methods) involves placement of a small amount of suspected allergy-causing substances (allergens) on the skin (usually the forearm, upper arm, or the back), and then scratching or pricking the skin so that the allergen is introduced under the skin surface. The skin is observed closely for signs of a reaction, usually swelling and redness of the site -- a controlled hive with so-called wheal and flare. Results are usually obtained within about 20 minutes, and several suspected allergens can be tested at the same time.

A similar method involves injection of a small amount of allergen under the surface of the skin (intradermal) and watching for a reaction at the site. Skin tests are most useful for respiratory allergies (in which the allergen is inhaled), a penicillin allergy, insect bite allergies, and food testing.

Food allergies are usually tested by using various elimination diets -- the suspected foods are eliminated from the diet for several weeks and then gradually re-introduced one at a time while the person is observed for signs of an allergic reaction. Because a person might believe he or she is allergic to a certain food, a double-blind test may be advised to find out if this is true. In a double-blind test, suspected foods and placebos (harmless substances) are given in a disguised form. The person being tested and the provider are both unaware of whether the substance tested in that session is the placebo or the food substance. A third party knows the identity of the substances and identifies them with some sort of code. This test requires several sessions if more than one substance is under investigation.

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