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Alternative Names Return to topPatch tests - allergy; Scratch tests - allergy; Skin tests - allergy; RAST test
Definition Return to top
Allergy tests are any of several tests used to determine the substances to which a person is allergic.
How the Test is Performed Return to top
There are many methods of allergy testing. Among the more common are:
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:
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.
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 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:
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 Return to top
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 Return to top
Skin tests may cause very mild discomfort when the skin is pricked. Itching may occur if you have a positive reaction to the allergen.
Why the Test is Performed Return to top
Allergy tests are done to determine the specific substances that cause an allergic reaction in a person.
They may also be done to determine if a group of symptoms is a true allergic reaction. Some food intolerances 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:
Normal Results Return to top
In a nonallergic person, allergy tests should be negative (no response to the allergen).
What Abnormal Results Mean Return to top
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 Return to top
Risks related to skin and food allergy tests may include:
Considerations Return to top
The accuracy of allergy testing varies quite a bit. Even the same test performed at different times on a person may give different results. A person may react to a substance during testing, but never react during normal exposure. Rarely, a person may also have a negative allergy test and still be allergic to the substance.
References Return to top
Wallace DV, Dykewicz MS, Bernstein DI, Blessing-Moore J, Cox L, Khan DA, et al. The diagnosis and management of rhinitis: an updated practice parameter. J Allergy Clin Immunol. 2008 Aug:122(2).
Kurowski K, Boxer RW. Food allergies: detection and management. American Family Physician. 2008 June:77(12).Update Date: 1/9/2009 Updated by: Stuart I. Henochowicz, MD, FACP, Associate Clinical Professor of Medicine, Division of Allergy, Immunology, and Rheumatology, Georgetown University Medical School. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.