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Allergy Tests

How To Prepare

Many medicines can affect the results of a skin test. Be sure to tell your doctor about all the nonprescription and prescription medicines you take. You may need to stop taking some medicines, such as some tricyclic antidepressants and antihistamines such as cetirizine (Zyrtec), fexofenadine (Allegra), and loratadine (Claritin), before you have an allergy skin test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will show. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

Skin tests

The health professional doing the skin prick or intradermal test will:

  • Clean the test site (usually on your back or arm) with alcohol.
  • Place drops of the allergens on your skin camera.gif about 1 in. (2.5 cm) to 2 in. (5 cm) apart. This allows many substances to be tested at the same time.
  • Prick the skin under each drop with a needle. The needle passes through the drop and allows some of the allergen to penetrate your skin. For the intradermal test, a needle is used to inject the allergen solution deeper into the skin.
  • Check the skin after 12 to 15 minutes for red, raised itchy areas called wheals. If a wheal forms, it means you are allergic to that allergen (this is called a positive reaction).

An alternative skin prick method uses a device with 5 to 10 points (heads), which are dipped into bottles that contain the allergen extract. This device is pressed against the skin of the forearm or back so that all heads are pressed into the skin at the same time.

If the skin prick test is negative, you may choose to have an intradermal skin test at a later visit. A skin prick test is usually done first because the intradermal test has a greater chance of causing a severe allergic reaction.

The skin prick test and the intradermal test usually take less than an hour each.

Skin patch test

A skin patch test also uses small doses of the suspected allergen. For this test:

  • Doses of allergens are placed on patches that look like adhesive bandages.
  • The patches are then placed on the skin (usually on your back). This usually takes about 40 minutes, depending on how many patches are applied.
  • You will wear the patches for 24 to 72 hours. Do not take a bath or shower or do any activities that could make you sweat excessively while you are wearing the patches. This could loosen the patches and cause them to fall off.
  • The patches will be removed by your health professional, and your skin will be checked for signs of an allergic reaction.

Blood test

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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