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Allergy skin tests
By Mayo Clinic staffMayo Clinic Health Manager
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Definition
During an allergy skin test, your skin is exposed to allergy-causing substances (allergens) and then is observed for signs of a local allergic reaction.
Along with your medical history, allergy tests can confirm whether signs and symptoms, such as sneezing, wheezing and skin rashes, are caused by allergies. Allergy tests can also identify the specific substances that trigger allergic reactions. Information from allergy tests can help your doctor develop an allergy treatment plan that may include allergen avoidance, medications or allergy shots (immunotherapy).
Why it's done
Allergy skin testing is widely used to diagnose allergic conditions such as:
- Hay fever
- Allergic asthma
- Dermatitis (eczema)
- Food allergies
- Penicillin allergy
- Bee sting allergy
Skin testing can be used for people of all ages, including infants and older adults. Sometimes, however, skin tests aren't recommended. Your doctor may advise against skin testing if you:
- Take medications that interfere with test results. These include antihistamines, many antidepressants and some heartburn medications. Your doctor may determine that it's better for you to continue taking these medications than to temporarily discontinue them in preparation for a test.
- Have a severe skin disease. If conditions such as eczema or psoriasis affect large areas of skin on your arms and back — the usual testing sites — there may not be enough clear, uninvolved skin to do an effective test.
- Are highly sensitive to suspected allergens. You may be so sensitive to certain substances that even the small amounts of them used in skin tests could trigger a severe allergic reaction (anaphylaxis).
Blood tests — technically called in vitro allergen-specific IgE antibody tests — are particularly useful for those who shouldn't undergo skin tests. Although blood tests can be as accurate as skin tests, they're not performed as often because they can be less sensitive and are more expensive. If you want to start immunotherapy, which is a series of injections intended to increase your tolerance to allergens, you need either a skin or blood test to identify the specific substances that trigger your allergies.
Risks
The most common side effect of skin testing is slightly swollen, red, itchy bumps (hives). Hives may be most noticeable during the test. They usually go away within a few hours, although they can persist for a day or two. A mild cortisone cream can be applied to relieve the itching and redness.
Rarely, allergy skin tests can produce a severe, immediate allergic reaction, so it's important to have skin tests performed at an office where appropriate emergency equipment and medications are available. If you develop a severe allergic reaction in the days after a skin test, call your doctor right away.
Some doctors who practice complementary or alternative medicine may perform provocation-neutralization tests, but these tests aren't proved and aren't considered reliable.
How you prepare
Before recommending a skin test, your doctor will ask detailed questions about your medical history, your signs and symptoms, and your usual way of treating them. Your answers can help your doctor determine if allergies run in your family and if you might also have them.
Next, your doctor will perform a physical examination to search for additional clues about the causes of your signs and symptoms.
Your medical history and physical examination may provide enough information for your doctor to discuss your diagnosis and treatment. If so, a skin test may be unnecessary. But if your doctor is uncertain or suspects that you have allergies and needs more information about the possible causes, he or she may recommend that you have a skin test.
Medications can interfere with results
Before scheduling a skin test, your doctor will need a list of all your prescription and over-the-counter medications. Some medications can suppress allergic reactions, preventing the skin testing from working effectively. Other medications may increase your risk of developing a severe allergic reaction during a test.
Because medications clear out of your system at different rates, your doctor may ask that you stop taking certain medications for up to 10 days. Medications that can interfere with skin tests include:
- Prescription nonsedating antihistamines, such as fexofenadine (Allegra)
- Over-the-counter antihistamines (Claritin, Benadryl, Chlor-Trimeton, Zyrtec, others)
- Tricyclic antidepressants, such as amitriptyline and doxepin (Sinequan)
- Heartburn medications, such as cimetidine (Tagamet) and ranitidine (Zantac)
What you can expect
Skin 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.
Results
Before you leave your doctor's office, you'll know the results of a puncture, prick or scratch test or an intradermal test. A patch test may take several days or more to produce results.
A positive skin test means that you may be allergic to a particular substance. Bigger bumps usually indicate a greater degree of sensitivity. A negative skin test means that you probably aren't allergic to that particular allergen.
The accuracy of skin tests can vary. You may react differently to the same test performed at different times. Or you may react positively to a substance during a test but not react to it in everyday life.
In general, skin tests are most reliable for diagnosing allergies to airborne substances, such as pollen, pet dander and dust mites. Because diagnosing food allergies can be complex, you may need additional tests or procedures.
Your allergy treatment plan may include medications, immunotherapy, environmental changes or dietary changes. Ask your doctor to explain anything about your diagnosis or treatment that you don't understand. With test results that identify your allergens and a treatment plan to help you take control, you'll be able reduce or eliminate allergy signs and symptoms.
- 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.
