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Ency. home > Test > A > Allergy testing

Allergy testing   

Overview | Risks | Results

Alternative names:

Patch tests - allergy; Scratch tests - allergy; Skin tests - allergy; RAST test

Definition:

Any of several tests, blood, protocols, or challenge, used to determine the substance(s) to which a person is allergic.

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, and insect bite allergies.

Food allergies are usually tested by using various "elimination" diets (in which the suspected food(s) 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 food allergies are often affected by what the person thinks they might be allergic to, a double-blind test may be advised. In this test, suspected foods and placebos (inert substances) are given in a disguised form. The person being tested and the health care provider are both unaware of whether the particular substance being tested in that session is a placebo or a food substance (a 3rd 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. 

While this strategy is useful and practical for mild allergic reactions, it must be undertaken carefully in individuals with suspected severe or anaphylactic reactions to foods.  In this instance, blood tests are the safer first approach, as described below. Of note, skin testing is almost never performed to detect food allergies for the same reason of the higher risk of causing a severe allergic reaction.

The RAST is a laboratory test performed on blood (see venipuncture). It tests for the amount of specific IgE antibodies in the blood (which are present if there is a "true" allergic reaction).

Other tests include immunoglobulin measurements (see serum immunoglobulin electrophoresis) and the blood cell differential and/or absolute eosinophil count (increased eosinophils can indicate the presence of allergy). Provocation (challenge) testing includes exposure to the suspected allergen (for example, in the diet or by inhaling the suspected allergen) under controlled circumstances. This type of test can provoke severe allergic reactions.

How to Prepare for the Test

Before any allergy testing, the health care provider will ask for a very detailed medical history. This may include questions about such things as illnesses, emotional and social conditions, work, entertainment, lifestyle, foods, and eating habits. 

If skin testing will be performed, it is essential to take no antihistamines prior to the test, as this may lead to false negatives, and hence false reassurance that a substance is unlikely to cause a severe allergic reaction.  Your doctor will provide the relevant information on which medicines to avoid and when to stop taking them prior to testing.

How the Test will Feel

Skin tests may cause very mild discomfort when the skin is scratched or pricked. Itching may accompany a positive reaction to the allergen.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

Why the Test is Performed

Allergy tests are used to determine the specific substances that cause an allergic reaction in an individual. They may also be used to determine if a group of symptoms is a true allergic reaction (involving antibodies and histamine release). Some food intolerances, in which there is an inability to digest a food because of lack of appropriate enzymes, mimic allergies. Some drugs, such as aspirin, can cause allergy-like symptoms but without the formation of antibodies or the release of histamine.

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