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By Mayo Clinic staffSkin testing is usually performed in a doctor's office. Typically, a nurse administers the test and a doctor interprets the results.
Allergy skin tests aren't painful. Because the needles used in these tests barely penetrate your skin's surface, you won't bleed or feel more than mild, momentary discomfort.
During the test
There are three main types of skin tests:
- Puncture, prick or scratch test (percutaneous). In this test, which is the most common type of skin test, tiny drops of purified allergen extracts are pricked or scratched into your skin's surface. This test is usually performed to identify allergies to pollen, mold, pet dander, dust mites and foods.
- Intradermal test (intracutaneous). Purified allergen extracts are injected into the skin of your arm. This test is usually performed if your doctor suspects that you're allergic to insect venom or penicillin.
- Patch test (epicutaneous). This test doesn't use needles. Instead, an allergen is applied to a patch, which is then placed on your skin. This test is usually performed to identify substances that cause contact dermatitis. These include latex, medications, fragrances, preservatives, hair dyes, metals and resins.
Some tests detect immediate allergic reactions, which develop within minutes of exposure to an allergen. Other tests detect delayed allergic reactions, which develop over a period of several days.
Tests for immediate allergic reactions
A puncture, prick or scratch test checks for immediate allergic reactions to as many as 40 different substances at one time. In adults, the test is usually done on the forearm. Children are usually tested on the upper back.
After cleaning the test site with alcohol, the nurse draws small marks on your skin and applies a drop of allergen extract next to each mark. He or she then uses a sharp instrument (lancet) to introduce the extracts into the skin's surface. A new lancet is used for each scratch to prevent cross-contamination of allergens. The drops are left on your skin for 15 minutes, and then the nurse observes your skin for signs of allergic reactions.
To see if your skin is reacting the way it's supposed to, the nurse introduces two additional substances into your skin's surface:
- Histamine. In almost everyone, this substance causes a skin response, so it's used as a positive control. If you don't react to histamine, the skin test may be difficult or impossible to interpret.
- Glycerin or saline. In almost everyone, these substances cause no reaction. So one or the other is used as a negative control. If you react to glycerin or saline, you may have sensitive skin, so your reactions to the allergen extracts will need to be interpreted with caution.
You may need a more sensitive immediate-reaction test — known as an intradermal test — if a puncture, prick or scratch test is inconclusive. During this test, a nurse uses a thin needle and syringe to inject a small amount of allergen extract just below the surface of the skin on your arm. Then he or she inspects the site after 15 minutes for a local skin reaction.
Tests for delayed allergic reactions
Patch tests detect delayed allergic reactions. During a patch test, your skin may be exposed to 20 to 30 extracts of substances that can cause an allergic skin reaction (contact dermatitis).
Allergen extracts are applied to patches that you wear on your arm or back for 48 hours. During this time, you should avoid bathing and activities that cause heavy sweating. The bandages are removed when you return to your doctor's office for an evaluation.
After the test
If an allergen causes an allergic reaction to a puncture or intradermal skin test, you'll develop a raised, red, itchy bump that may look like a mosquito bite. A nurse will then measure the bump's size.
After the nurse records the results, he or she will clean your skin with alcohol to remove the marks and allergen droplets. Then you'll meet with your doctor to discuss the results and possible treatment options.
- Tips to remember: What is allergy testing? American Academy of Allergy, Asthma and Immunology. http://www.aaaai.org/patients/publicedmat/tips/whatisallergytesting.stm. Accessed Feb. 24, 2009.
- Allergy testing. American College of Allergy, Asthma & Immunology. http://www.acaai.org/public/background/testing.htm. Accessed Feb. 24, 2009.
- Demoly P, et al. In vivo methods for the study of allergy. In: Adkinson NF, et al. Middleton's Allergy: Principles and Practice. 7th ed. St. Louis, Mo.: Mosby; 2008:1267.
