What is a skin test?
A skin test is a test done on the skin to identify if a substance (an allergen) is potentially triggering an allergic reaction. Environmental allergens (aeroallergens), including furry animals, dust mites, tree pollen, grass pollen, weed pollen, and molds can cause allergic eye symptoms, nasal symptoms, and asthma symptoms. Allergens that can trigger allergic reactions involving the skin, respiratory system, gastrointestinal system, and cardiovascular system include foods, medications, latex, and insect venoms.
How is an allergy skin test done?
Allergy skin testing is noninvasive and generally very well tolerated, even by small children. A small amount of the suspected allergy-provoking substance (the allergen) is placed on the skin. The skin is then gently punctured through the small drop with a special sterile puncture device. An allergy skin test is also called a prick/puncture test. The older terminology was "scratch test." With venoms, certain medications, such as penicillin, and in certain cases with aeroallergens, a second step of testing involves injecting a small amount of the allergen into the skin with a small needle. This is called an intradermal or an intracutaneous test. This type of testing should never be performed for foods.
What is a positive skin test result?
A positive skin test involves the formation of a bump (wheal) and redness (flare). In addition to the allergens in question, skin testing is also performed with a positive control (histamine) that should always cause a skin reaction, and a negative control, (saline), that should not cause a reaction. A test is positive if the allergen causes a wheal 3 mm greater than the negative control, and if the skin has a response to the histamine, as well. It is important to know that individuals cannot undergo skin testing if they are using antihistamines, since this blocks the histamine-mediated reaction.
For example, if a specific food allergy is suspected, a skin test uses a dilute extract of the suspected food. A small drop of this particular liquid extract is placed on the skin of the forearm or back. This underlying skin is gently punctured through the small drop with a special puncture device. The test is positive if there is a wheal and flare response described above. A positive test raises the possibility of a true allergy but is not diagnostic or confirmatory for having a true allergy. If there is no reaction with a wheal and flare, the test is read as negative and being allergic is very unlikely. If the skin test is positive, it implies that the patient has a type of antibody (IgE) on specialized cells in the skin that release histamine to cause symptoms of an allergic reaction. These cells are called mast cells and the IgE antibody bound to them is specific to the food being tested, like a puzzle piece. It is important to note that a positive skin test does not automatically mean a person is allergic to a food, medication, or aeroallergen. The skin test is one component of the information an allergist uses to make an accurate diagnosis, but the most important information comes from the patient's reported symptoms.
Diagnosing Food Allergy
Skin tests: In a percutaneous prick-puncture skin test, a dilute extract of the suspected food is placed on the skin of the forearm or back. This portion of the skin then is punctured or pricked with a needle and observed for swelling or redness, which would signify a local allergic reaction to the food. A positive prick-puncture test indicates that the patient has the IgE antibody that is specific for the food being tested on the skin's mast cells. Skin tests are rapid, simple, and relatively safe.
What are the advantages of skin tests?
Skin tests are rapid, simple, and relatively safe. They can be very helpful in specifically identifying causes of allergies.
Is there danger to a skin test?
In very rare cases, patients may have a severe allergic reaction involving multiple body systems to skin testing. For this reason, skin testing should be completed by an experienced allergist who is well prepared to diagnose and treat a severe allergic reaction. Although both are safe, intradermal or intracutaneous skin testing is more likely to cause a severe allergic reaction compared to prick/puncture testing.
What is done if a skin test can't be done?
For these patients, a doctor may use special blood tests, such as the RAST and the ELISA. These tests measure the presence of specific types of IgE in the blood.
These tests may cost more than skin tests, and results are not available immediately. As with skin testing, positive RAST and ELISA tests do not by themselves necessarily make the final diagnosis.
Allergies can best be described as:
Bernstein, et al. "Allergy Diagnostic Testing: an update practice parameter." Annals of Allergy, Asthma, and Immunology 100 (2008): S1-149.