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Allergy testing - skin

Definition

Allergy skin tests are tests used to find out which substances cause a person to have an allergic reaction.

Alternative Names

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

How the test is performed

There are three common methods of allergy skin testing.

The skin prick test involves:

  • Placing a small amount of substances that may be causing your symptoms on the skin, most often on the forearm, upper arm, or back.
  • Then, the skin is pricked so the allergen goes under the skin's surface.
  • The health care provider closely watches the skin for swelling and redness or other signs of a reaction. Results are usually seen within 15-20 minutes.
  • Several allergens can be tested at the same time.

The intradermal skin test involves:

  • Injecting a small amount of allergen into the skin. 
  • Then the health care provider watches for a reaction at the site.
  • This test is more likely to be used to find out if you are allergic to something specific, such as bee venom or penicillin.

Patch testing is a method to diagnose the cause of skin reactions that occur after the substance touches the skin.

  • Possible allergens are taped to the skin for 48 hours.
  • The health care provider will look at the area in 72 - 96 hours.

How to prepare for the test

Before any allergy testing, the health care provider will ask questions about:

  • Illnesses
  • Where you live and work
  • Lifestyle
  • Foods and eating habits

Allergy medicines can change the results of skin tests. 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 pricked.

You may have symptoms such as itching, a stuffy nose, red watery eyes, or a skin rash if you allergic to the substance in the test.

Rarely, people can have a whole-body allergic reaction (called anaphylaxis), which can be life threatening. This usually only occurs with intradermal testing. Your health care provider will be prepared to treat this serious response.

Why the test is performed

Allergy tests are done to determine what substances are causing your allergy symptoms.

Your doctor may order allergy skin tests if you have:

  • Hay fever (allergic rhinitis) and asthma symptoms that are not well controlled with medicine
  • Hives and angioedema
  • Food allergies
  • Skin rashes (dermatitis), in which the skin becomes red, sore, or swollen after contact with the substance
  • Penicillin allergy*
  • Venom allergy

*NOTE: Allergies to penicillin and closely related medicines 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.

Normal Values

A negative test result means there were no skin changes in response to the allergen. This negative reaction most often means that you are not allergic to the substance.

Rarely, a person may have a negative allergy test and still be allergic to the substance.

What abnormal results mean

A positive result means you reacted to a substance. Your health care provider will see a red, raised area called a wheal.

Often, a positive result means the symptoms you are having are due to exposure to that substance. In general, a stronger response means you are more sensitive to the substance.

People can have a positive response to a substance with allergy skin testing, but not have any problems with that substance in everyday life.

Skin tests are usually accurate. However, if the dose of allergen is large, even people who are not allergic will have a positive reaction.

Your health care provider will consider your symptoms and the results of your skin test to suggest lifestyle changes you can make to avoid substances that may be causing your symptoms.

References

Bernstein IL, Li JT, Bernstein DI, Hamilton R, et al. American Academy of Allergy, Asthma and Immunology; American College of Allergy, Asthma and Immunology. Allergy diagnostic testing: an updated practice parameter. Ann Allergy Asthma Immunol. 2008 Mar;100(3 Suppl 3):S1-148.

Demoly P, Bousquet J, Romano A. In vivo methods for the study of allergy. In: Adkinson NF Jr, ed. Middleton’s Allergy: Principles and Practice. 7th ed. Philadelphia, Pa: Mosby Elsevier; 2008:chap 71.

Gober MD, DeCapite TJ, Gaspari AA. Contact dermatitis. In: Adkinson NF Jr, ed. Middleton’s Allergy: Principles and Practice. 7th ed. Philadelphia, Pa: Mosby Elsevier; 2008:chap 63.


Review Date: 6/17/2012
Reviewed By: David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc., and Stuart I. Henochowicz, MD, FACP, Associate Clinical Professor of Medicine, Division of Allergy, Immunology, and Rheumatology, Georgetown University Medical School.
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