Skin-prick tests are fast and inexpensive, and can check for more than one allergy at a time. A small drop of liquid mixed with an allergen is placed on the skin — usually the arm — and then the skin is lightly pricked with a needle. An itchy red welt 20 minutes later indicates a possible allergy to the substances being tested.
A more sensitive option is intradermal testing, the test of choice for particular allergens like penicillin or bee venom, which may not be detected with a skin-prick test. Intradermal testing involves the injection of the suspected allergen between the layers of the skin. But both these tests can give false positives. “The strength of the skin tests are when they’re negative,” says Stuart Carr, a paediatric allergist and immunologist and president of the Canadian Society of Allergy and Clinical Immunology. “That’s when you can usually safely rule out an allergy.”
If a skin-prick or intradermal test result is positive, inconclusive or can’t be administered because of your child’s medications or eczema, a blood test might be used. Blood is checked for levels of specific antibodies, but these tests are more costly, aren’t covered in every province, take longer to produce results and can also produce false negatives. Plus, a blood draw isn’t exactly a fun fair for a kid.
An “open challenge,” in which your child is given the suspect food or medication, is the most accurate assessment of all. Blood and skin tests may take a couple of years to give a negative result after a child loses an allergy because they can still have antibodies against a specific allergen, even if they no longer react to it. “Real life is the ultimate test,” Roberts says.
That idea may make parents nervous, but under close medical supervision it can be done safely in a hospital or clinic. “In children where we believe they may have outgrown their allergy, a challenge is a supervised means of introducing it to them again.” Giving a kid real peanuts to eat, for example, is considered the gold standard in testing.
A version of this article appeared in our April 2012 issue, with the headline “Put Allergies to the Test.” p. 38.