Allergy tests detect the presence of antibodies to a particular allergen, or something that causes an allergic reaction. If a test is positive, this suggests that you may have an allergy sensitization to this allergen. A positive allergy test must correlate with symptoms you experience upon exposure to the allergen. We recommend that allergy testing be performed by a board-certified allergist.

TYPES OF SKIN TESTING

There are two general approaches to allergy skin testing - the prick and the injection methods. In the prick method, a drop of extract is introduced using a small sharp instrument, causing a small break in the skin. With the injection method, a drop of allergen extract is injected into the top layer of the skin, raising a small bubble on its surface.

Both of these tests are simple and inexpensive. The prick method has advantages in that it's safe, causes very little discomfort to the patient and allows medical personnel to test many allergens in one session.

On the other hand, the injection method tends to have more consistent results and is many times more sensitive in identifying allergens. It is so sensitive; however, that the test may identify insignificant allergens, or those that do not cause any symptoms. You can refer to the Allergy page for more details on allergy testing.

These tests are described in detail below.

HOW DOES THE TEST WORK?

In either method, the allergy extract causes a reaction in the skin in about 20 minutes. A negative reaction shows no change, while a positive reaction causes a small red welt to develop. The size of the welt is measured to determine the strength of the reaction. It's important to keep in mind that a positive result only indicates a possible allergy. Your doctor will review your skin test results, your medical history and physical exam to identify which allergens are causing you to experience symptoms.

WHAT FACTORS CAN AFFECT SKIN TESTS?

Location of Test
Doctors prefer the back for skin testing because it is a large and uniform area. In addition, the skin on the back tends to be more sensitive to the testing, thus giving better results.

Medications
The use of antihistamine medications may affect skin tests. Therefore, we generally recommend that you discontinue taking an antihistamine three to seven days before skin testing is done. Hismanal (Astemizole®), a non-sedating antihistamine, must be discontinued for two to three months prior to skin testing. Also, some antidepressants affect skin tests and may need to be discontinued before testing. Steroid treatment does not affect skin testing and need not be discontinued before skin testing.

Standard skin tests are the best and most reliable method of testing for allergies. These tests produce immediate results and are simple and inexpensive. Skin tests are not the final step in the diagnostic process, but serve as a guide for further investigation. Skin tests alone never prove the presence of allergies, symptoms after exposure must also be present.

The prick method is the most common type of skin test. A drop of allergen extract is placed on the skin and a tiny prick is made into the skin. If the patient has IgE antibodies to the allergen, a hive will appear at the site within about 15 minutes. Many allergens can be tested at the same time. This method is safe and causes very little discomfort.

The intradermal test is another type of skin test. A syringe is used to inject allergen extract into the top layer of skin, which raises a small bubble on the surface. The result is similar to the prick test. A hive will appear in about 15 minutes if IgE antibodies are present. Intradermal tests are usually placed on the upper arm. The intradermal method is many times more sensitive in identifying allergies than the prick test, but this can be a disadvantage. The test may show positive for allergens that don’t cause any significant problems for the patient under real-life circumstances. (This can happen with prick tests too, but not as often.)

A third kind of skin test, called a patch test, can be used to look for substances that might be causing contact dermatitis. A suspected allergen, such as a cosmetic, is placed on the skin and covered with a bandage for 48 hours. If the patient is allergic, the skin will become red and peel.

Antihistamines interfere with prick and intradermal skin tests. Before being tested, it’s necessary for the patient to stop taking such drugs for a period of time specified by the physician.

The radioallergosorbent (RAST) test, a blood test done in a laboratory, can be used instead of skin tests to detect allergies. The test is valuable in some situations. Some skin conditions might make skin testing impossible, or the physician might suspect that a patient is so sensitive to an allergen that a skin test could be dangerous. An advantage of the RAST test is that it’s not affected by antihistamines or other medications that the patient may be taking.

Positive skin or RAST tests help narrow the list of suspected allergens, but don’t often point directly to the cause of symptoms. The patient’s history is vital in determining which of the positive results really make sense. A challenge test may also be needed. If the patient’s allergic reactions haven’t been severe, the physician might deliberately expose the patient to one or more substances that produced a positive skin or RAST test. This is particularly true with food allergies.

SKIN TESTING FOR FOOD ALLERGIES

Diagnosing a food allergy usually requires a thorough review of your medical history, physical examination, skin testing, food elimination and sometimes, food challenge. A positive skin test indicates that you may have symptoms because of a particular food. However, a positive skin test result must correlate with your symptoms upon eating the food. Blood tests may also be performed to check for the presence of food allergies. Food allergies tend to be more common in children than in adults.