Allergy testing Health Article

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

How the test is performed

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

• Skin tests
• Elimination-type tests
• Blood tests (including the radioallergosorbent, or RAST, test)

SKIN TESTS

Skin tests are the most common. Specific methods vary.

One of the most common methods is the prick test. This test 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 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 seen within 20 minutes. Several allergens can be tested at the same time.

A similar method involves injecting a small amount of allergen into the skin and watching for a reaction at the site. This is called an intradermal skin test.

Skin tests are most useful for diagnosing:

• Insect bite allergies
• Mold, pollen, animal, and other allergies that cause allergic rhinitis and asthma
• Penicillin allergy

Allergies to penicillin and closely related medications are the only drug allergies that can be tested using skin tests. Skin tests for allergies to other drugs can be dangerous.

The prick skin test may also be used to diagnose food allergies. Intradermal tests are not used to test for food allergies because of high false positive results and the danger of causing a severe allergic reaction.

ELIMINATION TESTS

An elimination diet can be used to check for food allergies. An elimination diet is one in which foods that may be causing symptoms are removed from the diet for several weeks and then slowly re-introduced one at a time while the person is watched for signs of an allergic reaction.

Another version of this diet is the double-blind test. This method involves giving 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.

BLOOD TESTS

Blood tests can be done to measure the amount of immunoglobulin (Ig) E antibodies in the blood. This test may be used when skin testing is not helpful or cannot be done

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.