Hypersensitivity skin tests, both immediate and delayed, are useful for detecting allergies to airborne particles, foods, insect stings, penicillin and other substances. The most common, least expensive types of skin tests used for children include:
Percutaneous and intradermal skin tests. These immediate-type skin tests are administered by applying a diluted allergen to a prick or a scratch in the top layer of the skin (percutaneous or scratch test) or by using a very thin needle to inject the diluted allergen into the skin (intradermal test). Both are considered extremely safe and relatively accurate. Percutaneous skin testing is rarely conducted on infants younger than 6 months old; otherwise there is no age limit.
Intradermal testing, which is more sensitive, is used if the allergist strongly suspects a venom or penicillin allergy that was not detected by a percutaneous test. Intradermal tests, though low risk, can cause anaphylaxis in highly sensitive patients. It’s therefore important to be sure that the allergist’s office is prepared to treat life-threatening reactions, which can include a sudden drop in blood pressure, trouble breathing and a rapid increase in heart rate.
The accuracy of these tests can be undermined if children are taking certain medications, such as antihistamines, antidepressants or high-dose long-term steroids. Use of these drugs should be stopped, depending on the medication, three to 14 days before the day of testing. (Asthma medications or short bursts of oral steroids will not affect the results.)
After either type of test is administered, the tested area of the skin is observed for about 15 minutes to see if a reaction develops. A wheal (a raised, red and itchy bump) indicates the presence of the allergy antibody when the child comes in contact with a specific allergen — the larger the wheal, the greater the sensitivity.
Patch tests. These delayed hypersensitivity skin tests can be used to diagnose allergic contact dermatitis that flares when children are exposed to substances like rubber, fragrances or certain metals. Patients are asked to leave the patch test in place for 48 hours and keep it dry. An allergist will check the skin for reactions at specific times after the patch is removed.