Many common types of allergies can be confirmed with the use of skin testing.  But many people may not realize how and why skin testing is used, or think that this procedure is painful and inconvenient.  In truth, skin testing is now more accurate than ever, and it need not be painful or an ordeal.

Skin testing is used to diagnose immediate-type hypersensitivity allergy. This is the most common type of allergic reaction. In this type of allergy, symptoms occur very quickly after you are exposed to certain allergens. The body responds to allergens by producing certain kinds of proteins, called immunoglobulin E (IgE) antibodies. Antibodies are proteins that the body produces to fight off foreign invaders. One way to test a person for allergies is to measure the level of IgE antibodies in the blood. A simpler, more definitive way is to see whether exposure to certain allergens provokes a local response. This is the purpose of skin testing.

How is Skin Testing Done?
Skin testing involves the use of allergens—substances that provoke an allergic response. These might be types of pollen found in your area, as well as dust mites, mold, cockroach debris, animal hair and food. Extracts are made of common allergens such as these and are placed just beneath the surface of the skin. After a certain amount of time has elapsed, a person who is sensitive to a substance will have a red, raised bump appear on their skin where certain extracts were placed. This positive result as well as the findings of the physical examination and medical history are used by your doctor to identify whether you are allergic to that substance. A positive skin test by itself does not confirm whether the substance is causing your symptoms. Skin testing may be done by one of the two methods described below.

The Percutaneous Method. In one method of skin testing, a drop of each allergen extract is placed on the skin. Then the topmost layer of skin under the drops is lightly broken by pricking, puncturing, or scratching.This allows the extract to seep into the skin. Sometimes the drops are placed directly on one or more needles or clusters of needles. This method, called the percutaneous method, is usually done on the patient's lower arm or back.

The Intercutaneous Method. The second method of skin testing involves injecting the allergen extract under the first few layers of skin. This is done with a syringe and a very fine needle. 

One of the advantages of this method over the percutaneous method is that it is more sensitive—more often positive. It has the drawback, though, of yielding more false-positive results than the percutaneous method. A false-positive result is one that shows a reaction to an extract when the patient does not develop any symptoms on exposure to that allergen.

What Do the Results Show?

Once the allergen extracts have been placed on the skin, the doctor or nurse looks for signs of an allergic response. This appears in the form of a red, raised lump, or wheal, surrounded by a red, inflamed area, or flare. The exact size of the wheal and flare shows how allergic you are to that allergen. Usually, with a percutaneous test, the raised area must be at least 3 mm (about one-tenth of an inch) across to indicate a positive reaction. With an intracutaneous test, an even larger reaction is required.

A positive test only shows that you are allergic to the allergen. It does not necessarily mean that you will have symptoms on exposure to that allergen, or that allergen is a cause of your symptoms. Indeed, you may have positive skin tests, and your disease may be non-allergenic.

Since they are less sensitive, positive percutaneous skin tests are more closely related with your actually having symptoms (clinical sensitivity) than are the more sensitive intracutaneous tests. That is why it is important to have a physician with training in allergy performing and interpreting your skin tests.

Is Skin Testing Accurate?

Skin testing methods are more reliable now than in the past. At one time, it was found that the extracts used in skin testing varied greatly in how active they were in producing an allergic response. Even extract samples labeled as having the same strength were found to have different levels of activity. This finding greatly reduced the usefulness of skin testing in the opinion of many doctors.

Today, these problems have been corrected to a large extent. Now, standardized extracts have been developed for many allergens. Efforts are still underway to improve the standardization of food allergen extracts. Extracts made from these allergens tend to be less stable than other types of extracts.

How Can Skin Testing Help Me?

There is no reason to dread skin testing. This is an easy method to find out what specific substances are causing your allergies. It takes little time and causes only minor discomfort.

By pinpointing the specific allergens causing your allergy symptoms, your doctor can tailor your treatment to your particular case. For instance, if you have allergies only during a certain time of year, you may have an allergy to a specific kind of grass pollen. You may be able to use a medication to control your symptoms during the season when this grass is blooming. If you are allergic to substances that are present year round, such as dust or animal hair, you can take steps to limit your exposure to those allergens.

In other cases, your doctor may advise immunotherapy, or allergy shots. This treatment uses a serum containing minute amounts of the specific substances to which you are known to be allergic. Over time, your immune system's response to these allergens can be lessened by getting injections at regular intervals.

Whatever course of action you and your doctor decide to take, skin testing can provide valuable information about how your body responds to allergens. Armed with this knowledge, together you and your doctor can work out a treatment plan to keep your allergies under control.

SOURCE: This information should not substitute for seeking responsible, professional medical care. First created 1995; fully updated 1998; most recently updated 2005.
© Asthma and Allergy Foundation of America (AAFA) Editorial Board