Allergy skin testing is a safe and simple procedure that can yield useful information about your allergic sensitivities.
For more than a century, doctors have used skin tests to diagnose allergies. During these tests, your skin is exposed to allergy-causing substances (allergens) and then is observed for signs of an allergic reaction.
Along with your medical history, skin tests can confirm whether signs and symptoms such as sneezing, wheezing and skin rashes are caused by allergies. They can also identify the specific substances that trigger allergic reactions. Such information can help your doctor develop an allergy treatment plan that may include allergen avoidance, medications or allergy shots (immunotherapy).
Who is it for?
Skin testing is widely used to diagnose allergic conditions such as hay fever, allergic asthma and dermatitis (eczema). It's safe 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 conduct 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).
If you want to start immunotherapy - a series of injections intended to increase your tolerance to allergens - you need either a skin or blood test to identify the substances that trigger your allergies. Blood tests (in vitro allergen-specific IgE antibody tests) are particularly useful for those who should not undergo skin tests. Although blood tests can be as accurate as skin tests, they're not performed as often because they may be less sensitive and are more expensive.
How do 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 that you have allergies, 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.
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 so that the skin testing can't be performed. 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 10 days or more. Medications that can interfere with skin tests include:
- Prescription nonsedating antihistamines, such as fexofenadine (Allegra) and cetirizine (Zyrtec)
- Over-the-counter antihistamines (Benadryl, Chlor-Trimeton, others)
- Tricyclic antidepressants, such as amitriptyline and doxepin (Sinequan)
- Heartburn medications, such as cimetidine (Tagamet) and ranitidine (Zantac)
How is it done?
Skin testing is usually performed in a doctor's office. Typically, a nurse administers the test and a doctor interprets the results.
The three main types of skin tests are:
- Puncture, prick or scratch test (percutaneous). In this test, which is the type of skin test most commonly performed, 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, foods, insect venom and penicillin.
- 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). 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.
What can you expect during a skin test?
Contrary to what you may have heard, skin tests cause little if any discomfort. Because the needles used in these tests barely penetrate your skin's surface, you won't bleed or feel more than mild, momentary pain.
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 contact dermatitis. Caustic substances - such as industrial solvents - are diluted to prevent skin damage.
Allergen extracts are applied to bandages that you wear on your arm or back for 48 hours. During this time, you should avoid bathing and sweating heavily. The bandages are removed when you return to your doctor's office for an evaluation.
Before you leave the 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.
If an allergen provokes an allergic reaction to a puncture or intradermal skin test, you'll develop a raised, red, itchy bump (wheal and flare) 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.
A positive skin test means that you may be allergic to a particular substance. Bigger bumps usually indicate a higher 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 they're somewhat less reliable for diagnosing food allergies, you may need additional tests or procedures.
The most common side effect of skin testing is itching and redness. This may be most noticeable during the test, when you aren't allowed to scratch yourself. It usually subsides within a few hours, although it can persist until the next day. A mild cortisone cream can be applied to relieve the itching and redness.
Rarely, 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.