Allergy testing

• 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 15-20 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. It is more likely to be used when testing is being done to find out if you are allergy to something specific, such as bee venom or penicillin.

  Patch testing is a method to diagnose allergic reactions on the skin. Possible allergens are taped to the skin for 48 hours. The health care provider will look at the area in 24 hours, and then again 48 hours later.

  Skin tests are most useful for diagnosing:

  • Food allergy
  • Mold, pollen, animal, and other allergies that cause allergic rhinitis and asthma
  • Penicillin allergy*
  • Venom allergy
  • Allergic contact dermatitis

  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 method 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 to a specific allergen 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
  • Total IgE level
  • 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 pricked. Itching may occur if you have a positive reaction to the allergen.

• Why the test is performed

  Allergy tests are done to determine the specific substances that cause an allergic reaction in a person.

  Your doctor may order allergy tests if you have:

  • Allergic rhinitis and asthma symptoms that are not easily controlled with medications
  • Angioedema and hives
  • Food allergies
  • Contact dermatitis
  • Penicillin allergy*

  *NOTE: 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.

• Normal Values

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

• What abnormal results mean

  A positive result means you reacted to a specific substance. Often, but not always, a positive result means the symptoms that you are having are due to exposure to the substance in question. In general, a stronger response means you are more sensitive to the substance.

  People can have a positive response with allergy skin testing, but not have any problems with the specific substance in every day life.

  The skin tests are generally reliable. However, if the dose of allergen is excessive, a positive reaction will occur even in persons who are not allergic.

• What the risks are

  Risks related to skin and food allergy tests may include:

  • Allergic reaction
  • Life-threatening anaphylactic reaction
• Special considerations

  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

  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: 5/25/2009

Reviewed 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.