by Christopher C. Chang, M.D., Ph.D.
Your daughter pets a bunny and you notice her eyes get puffy. Your son eats shrimp dip and later complains of an itching sensation in his throat. Is it an allergy? Maybe, maybe not. In order to be certain, we perform allergy tests.
Rashes and hives worry parents. We look for triggers in substances which may have been ingested, inhaled or that have come in contact with the skin. When the culprit is poison ivy, for example, it is obvious and there’s no need to test for it. But environmental and especially food allergies can cause a variety of reactions, from the mild cases to full-blown life-threatening anaphylaxis. That is why it is a good idea to proceed with testing when allergies are suspected.
At the Nemours/Alfred I. duPont Hospital for Children, allergists are essentially testing when there is uncertainty. There is no single diagnostic test that tells us conclusively whether or not an allergy is present, as a glucose (blood sugar) test does for diabetes. The immune system is very complex and involves many pathways that result in allergies. Therefore, it is not possible to rely on one test to determine positivity or negativity. This is particularly true for food allergies.
Skin tests first. If we suspect an allergy, we start with a skin test. This involves pricking or scratching the skin with a tiny amount of liquid extract of an allergen (such as pollen or food). If the area swells up and becomes red the test is said to be positive, meaning that the child is allergic to that substance. Skin testing can be interpreted within 20 minutes, allowing the doctor to identify if a child has sensitivity to the substances tested during the office visit. For example, if we’re testing for pollen and the child develops a red blotch as big as their fist, that’s a strong reaction and can help us determine treatment. Skin testing is usually sufficient to indicate the presence or absence of environmental allergens. The test is somewhat less reliable for food allergies.
Blood tests when needed. In some cases, a child will show no reaction to the skin test, but that does not definitively mean they don’t have an allergy. This is especially true for food allergies. If symptoms such as a rash, swelling or tingling/burning around the mouth have been observed in connection with a suspected food, sometimes we will proceed with blood testing in spite of the negative skin test. In the blood test, the presence of high amounts of a class of immunoglobulins known as IgE suggests that an allergy is likely. It’s important to note that most food intolerances are not allergic reactions. For instance, many children experience stomachaches, nausea, diarrhea, and headaches that are not caused by allergies but may be due to lactose or MSG intolerance. Patients should also be aware that there are many places that offer certain “blood tests” that have no relevance to food allergies at all.
Challenge tests when other results aren’t conclusive.Ultimately, the most definitive test for food allergies is the challenge test where we introduce small amounts of the suspected allergenic substance, shellfish for example, and watch for a reaction. We will often do this when the skin and blood tests are negative or become less reactive. Our objective is to challenge to confirm absence of a food allergy, so we select those patients most likely to pass a challenge based on history and testing. A challenge test is about a half-day affair where the amount of the food in question is gradually increased over time and the child is monitored closely for any adverse effect. Dangerous reactions usually occur within the first half hour of ingestion. If there is no reaction during the challenge, the suspected allergy has been ruled out and the child is free to consume the food and is thought to be at no greater risk than the general population
Christopher C. Chang, M.D., Ph.D., is division chief of the division of pediatric allergy/immunology at Nemours/Alfred I. duPont Hospital for Children, and clinical professor of pediatrics at Jefferson Medical College.
Does your child have an allergy? What tests confirmed it?