Allergy testing

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

Allergy tests are 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, insect bite allergies, and food testing.

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 may be a 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. Challenge testing should only be done by a physician.

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, mimicallergies. Some drugs, such as aspirin, can cause allergy-like symptoms but without the formation of antibodies or the release of histamine.

Normal Values

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

What abnormal results mean

Most often, a positive test indicates an allergy 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.

Additional conditions under which the test may be performed:

What the risks are

Skin tests and Food allergy tests carry a risk that you will experience an allergic reaction when exposed to the allergens. Some sensitive persons will (rarely) experience a life-threatening anaphylactic reaction.

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).

Tests that are not proven as valid include cytotoxic testing (in which foods are mixed in a test tube with a sample of the person’s blood and then the white blood cells are observed for a change in shape) and “provocation and neutralization” testing (in which the person is injected with a food or other substance and, if there is an allergic reaction, they are injected with more of the substance to “neutralize” the response; this can be very dangerous!).

Special considerations

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

Johns Hopkins patient information

Last revised: December 6, 2012
by Dave R. Roger, M.D.

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