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Allergy Tests

Results

A skin or blood test can tell you what substance, or allergen, may trigger an allergic response.

Skin tests

Skin tests work by exposing a person to suspected allergens and seeing if a reaction occurs. The results of the skin tests will be available immediately after testing is done.

Allergy skin tests

Normal (negative):

No raised red areas (called wheals) are created by the allergen.

Abnormal (positive):

A wheal created by the allergen is at least 1/8 inch (3 mm) larger than the reaction to the negative control. The larger the wheal, the more certain it is that the person is allergic to that specific allergen.

Blood tests

Allergy blood tests look for substances in the blood called antibodies. Results of allergy blood tests are usually available in about 7 days.

Allergy blood tests

Normal (negative):

The levels of immunoglobulin E (IgE), a type of antibody, are the same as in a person who does not have allergies.

Abnormal (positive):

The levels of immunoglobulin E (IgE) antibodies for a particular allergen or group of allergens are 4 times the normal level.

What Affects the Test

Reasons you may not be able to have a skin test or why the results may not be helpful include:

What To Think About

  • You may not need allergy testing if you have mild allergies that are easily controlled with medicine or lifestyle changes.
  • Skin tests for allergies:
    • Are the easiest and least expensive method for identifying allergies in most people.
    • Are more reliable than blood tests for identifying common inhaled (airborne) allergies, such as pollens, dust, mold, and pet dander.
    • Cannot tell whether a person is reacting to a certain food. Further testing (such as an elimination diet) may be needed to identify this.
    • Can be unpleasant for children and their parents.
    • May take as long as 3 to 4 hours.
  • Blood tests for allergies:
    • Are less sensitive than skin tests. So it is possible for people to have a positive reaction to a skin test but have a negative reaction to a blood test.
    • Are not affected by antihistamines or tricyclic antidepressants, so people do not need to stop taking these medicines while being tested.
    • May be a good choice if you have very sensitive skin or a skin problem, such as eczema, that would make it hard to see whether you have a reaction to a skin test.
    • Will not cause an allergic reaction. They may be used if you have had a serious allergic reaction in the past.
    • Cost more than skin tests.
  • You may need to be tested for a latex allergy before having a major surgery if you have had frequent exposure to latex. Allergy blood tests are used for this. If you have a latex allergy, latex products should not be used during your surgery or recovery.
  • Other blood tests, such as fluoro-allergosorbent testing (FAST), multiple antigen simultaneous testing (MAST), and radioallergosorbent testing (RAST), are sometimes used to check for allergens.

WebMD Medical Reference from Healthwise

Last Updated: June 30, 2011
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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