Skin testing is the most sensitive and least expensive way to identify specific allergies. Newer ways of performing the skin test minimize patient discomfort.
Scratch tests may be used on people of any age to screen for allergies and detect those individuals who are super-sensitive. To perform the scratch test, a drop of allergen solution is placed on a disposable plastic device with 8 prongs. Each prong has a different allergen so that several allergens can be tests at once. The device is then pressed on the skin of the back.
Within 15-20 minutes, a red, itchy welt develops at the test site if the person is allergic to a particular allergen. Scratch tests can identify sensitivity to the pollens of trees, grasses and weeds, and to dust, mold spores, animal danders, insect venoms, foods, local anesthetics and some antibiotics.
Depending on patients’ specific problems, we generally use scratch tests in our office to screen for 32 different environmental allergens, and use a separate test for 24 different food groups.
Scratch testing may be sufficient for highly sensitive people, but many patients also require selected intradermal tests.
Intradermal testing consists of injecting a small drop of allergen solution between the upper layers of skin. A tiny needle is used to raise a small bubble on the skin of the arm. A welt around the injection site after 15-20 minutes indicates a positive reaction. About half of our patients also develop delayed reactions, with a welt reappearing 48-72 hours later.
Intradermal tests are more sensitive and more specific than scratch tests, especially for people with mild to moderate allergies. It is common for patients to have little or no reactions on scratch tests, yet be markedly positive with intradermal ones.
When standard intradermal test are inconclusive, we will sometimes do selected special intradermal tests with a more concentrated allergen solution.
In all these allergy test, the skin serves as a mirror to the body’s allergic sensitivity. While we would like to test the lungs in an asthmatic, or test the nose in someone with hayfever, the skin is a more accessible and convenient target organ.