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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

How the Test is Performed

There are many methods of allergy testing. Among the more common are:

• Skin tests
• Elimination-type tests
• Radioallergosorbent test (RAST)

SKIN TESTS

Skin tests are the most common. Specific methods vary. The scratch test, one of the most common methods, involves placing a small amount of suspected allergy-causing substances on the skin, usually the forearm, upper arm, or the back. Then, the skin is scratched or pricked so the allergen goes under the skin's surface. The health care provider closely watches the skin for signs of a reaction, usually swelling and redness of the site. Results are usually obtained within about 20 minutes, and a number of allergens can be tested at the same time.

A similar method involves injecting a small amount of allergen under the surface of the skin and watching for a reaction at the site.

Skin tests are most useful for diagnosing:

• Insect bite allergies
• Penicillin allergy
• Respiratory allergies

Penicillin and closely related medications are the only drugs that can be tested using skin tests. Skin tests for other drugs are at best non-informative and can be dangerous.

ELIMINATION TESTS

An elimination diet can be used to check for food allergies. An elimination diet is one in which the suspected foods are avoided for several weeks and then gradually re-introduced one at a time while the person is observed for signs of an allergic reaction.

Another version of this diet is the double-blind test. This method involves giving suspected foods and harmless substances in a disguised form. The person being tested and the provider are both unaware of whether the substance tested in that session is the harmless substance or the suspected food. 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.

While the double-blind strategy is useful and practical for mild allergic reactions, it must be done carefully in individuals with suspected severe reactions to foods. Blood tests may be a safer first approach. Skin testing is almost never performed to detect food allergies because of the higher risk of causing a severe allergic reaction.

BLOOD TESTS

RAST measures the amount of specific IgE antibodies in the blood. These antibodies are present if there is a true allergic reaction.

Other blood tests include:

• Absolute eosinophil count
• Blood differential
• Serum immunoglobulin electrophoresis

PROVOCATION

Provocation (challenge) testing involves exposing a person to a suspected allergen under controlled circumstances. This may be done in the diet or by breathing in the suspected allergen. This type of test may provoke severe allergic reactions. Challenge testing should only be done by a doctor.

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, you should NOT take antihistamines before the test. This may lead to a false-negative result, falsely reassuring you that a substance is unlikely to cause a severe allergic reaction. Your doctor will tell you which medicines to avoid and when to stop taking them before the test.

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.

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, which involves antibodies and histamine release. Some food intolerances, in which there is an inability to digest a food because of lack of appropriate enzymes, produce symptoms similar to allergies. Some drugs, such as aspirin, can cause allergy-like symptoms without the formation of antibodies or the release of histamine.

Additional conditions under which the test may be performed:

• Allergic rhinitis
• Angioedema
• Contact dermatitis
• Nasal obstruction

Normal Results

In a nonallergic person, allergy tests should be negative (no response to the allergen).

What Abnormal Results Mean

Most often, a positive test means you are allergic to the substance in question. The skin tests are most reliable when testing for airborne substances (such as animal dander or pollen). However, if the dose of allergen is excessive, a positive reaction will occur even in persons who are not allergic.

Risks

Risks related to skin and food allergy tests may include:

• Allergic reaction
• Life-threatening anaphylactic reaction

Considerations

The accuracy of allergy testing varies quite a bit. Even the same test performed at different times on a personmay give different results. A personmay react to a substance during testing, but never react during normal exposure. A personmay also have a negative allergy test and yet still be allergic to the substance.

References

Adkinson NF Jr. Middletons Allergy: Principles and Practice. 6th ed. Philadelphia, Pa: Mosby; 2003.

Rakel RE. Textbook of Family Medicine. 7th ed. Philadelphia, Pa: WB Saunders; 2007.

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