What is Allergy Testing?
A medical history combined with blood or skin allergy tests can determine if signs and symptoms, such as sneezing, itching or skin rashes are caused by allergies. By identifying the specific substances that can trigger allergic reactions, your physician can develop an allergy treatment plan that may include allergen avoidance, medications or allergy shots (immunotherapy) or drop (sublingual immunotherapy).
Who is allergy testing for?
Skin testing is used to diagnose allergic conditions such as hay fever, allergic asthma and dermatitis or eczema. Allergy testing is safe for all ages, including small infants and the elderly.
Skin tests aren’t for everyone. Your physician may not recommend skin testing if you:
- Take medications that could interfere with test results including antihistamines, some antidepressants and heartburn medicines. It may be better for you to continue taking those medications instead of temporarily discontinuing them in preparation for a test.
- Suffer from 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 extremely 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 or in vitro allergen-specific IgE antibody tests are particularly useful for those who should not undergo skin tests. 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 specific substances that trigger your allergies.
How do you prepare for an allergy skin test?
First, your physician will ask detailed questions about your medical history, signs, symptoms, and the typical way you treat them.
Your medical history and physical examination may provide enough information for your physician to discuss your diagnosis and treatment. If your physician suspects that you have allergies and needs more information about the possible causes, he or she may recommend that you have a skin test or blood test.
Before scheduling a skin test, your physician 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.
How is an allergy skin test done?
Skin testing is usually performed in a physician’s office. 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 physician 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 an allergy 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 discomfort.
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 activities that cause heavy sweating. The bandages are removed when you return to your physician’s office for an evaluation.
Results of an allergy testing on the skin
A puncture, prick or scratch test or an intradermal test will show results before you leave the office. A patch test may take longer to produce results.
If you have an allergic reaction to a puncture or intradermal skin test, you’ll develop a raised, red, itchy bump that looks like a mosquito bite.
After the results are recorded, we will clean your skin with alcohol to remove the marks and allergen droplets. Then you meet with the physician to discuss the results and treatment options.
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.
Risks of allergy testing on the skin
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.