What it is. Skin testing is the easiest, fastest, most sensitive, and, in many cases, the most cost-effective way to test for allergies. There are 2 types of skin tests: the scratch test and the intradermal test.
How a scratch test is done. Scratch tests involve applying a drop of allergen, such as dust mites, pet allergens, or pollens and molds, to tiny pricks or punctures made on the skin. Depending on how many allergens to which the doctor thinks you're allergic, he or she may perform as many as 20 or more scratch tests.
How intradermal testing is done. Intradermal testing involves injecting small amounts of diluted allergens under the top layers of the skin. Depending on how many allergens to which the doctor suspects you're allergic, he or she may perform as many as 20 or more tests.
What the results mean. A positive reaction is called a wheal and flare response. Within 10 to 15 minutes of application, the result is itchy, red, swollen areas that look like mosquito bites. The greater your reaction - the redder and more swollen the area is - the more allergic you are to the substance and the more likely you are allergic to it in the real world. While the size of the wheal can give your doctor important diagnostic information, a positive reaction by itself doesn't mean that particular allergen is the cause of your symptoms. On the other hand, if no reaction appears, the test is negative, and you're not allergic to that specific substance.
The benefits. Allergy experts say that intradermal tests are more sensitive than scratch tests. However, intradermal tests are also more likely to result in false positives. That means you have a positive test reaction but you don't have symptoms in everyday life. While the different types of allergy tests provide clues to your condition, your doctor's experience and expertise also come into play in interpreting the test results and how they apply to you.
The precautions. Skin testing is a less expensive and more sensitive way to test for allergies. However, a very few people, such as those who have widespread skin conditions such as eczema or who take medications that interfere with skin testing, such as antihistamines, can't use it. Antihistamines, found in over-the-counter allergy medications as well as in prescription allergy drugs, can interfere with skin testing. Many over-the-counter sleeping pills also contain an antihistamine. To tell, look for the name on the label. If you're taking an antihistamine, ask your doctor or pharmacist how long prior to skin testing you should stop taking it.