VANCOUVER, B.C. – The “shotgun” style of skin prick testing in children and adolescents with suspected IgE-mediated food allergy shows sensitization, but not necessarily allergy, according to Dr. James Bergman.
A positive skin test measures the presence of a specific IgE antibody, which does not necessarily equate to an allergy. Consequently, children may have multiple positive skin prick tests yet clinically tolerate the tested food, he said. “Sensitization is just the presence of a specific IgE to a food. Allergy is sensitization plus signs or symptoms upon exposure to the food.”
Dr. James Bergman
Dr. Bergman, who also holds a faculty position in the department of dermatology and skin science at the University of British Columbia, said the practice of shotgun skin prick testing can lead to unnecessary avoidance of specific foods. One group of researchers conducted oral food challenge tests in 125 children aged 1-9 years with a diagnosis of food allergy based on IgE tests. Nearly all of them (93%) had no reactivity when challenged with the suspect food (J. Peds. 2011; 158[4]:578-83). “Ninety-three percent of the children would have been avoiding their ‘allergic foods’ perhaps indefinitely,” said Dr. Bergman, who was not involved with the study.
“The general rule is, if you’re not having clinical symptoms that suggest an IgE-mediated reaction, then don’t test,” Dr. Bergman, a dermatologist who practices in Vancouver, said at the annual meeting of the Pacific Dermatologic Association.
“I explain to parents that if they want to test for a food in the situation where there is no IgE-mediated reaction, then it can be done, but there is a significant risk of a false positive or ‘fake allergy,’ ” he said. “In this situation the only way of knowing for sure whether it is an allergy is to undertake a formal oral food challenge, which is the (highest) standard for diagnosing food allergy.”
Telltale symptoms of an IgE-mediated food allergy include hives, vomiting, diarrhea, breathing problems, and change in level of consciousness. “These symptoms typically occur within minutes of ingestion, sometimes within 30 minutes and rarely up to 2 hours,” Dr. Bergman said. “If it’s beyond 2 hours, it’s unlikely to be IgE mediated.”
“If someone has a true food allergy, advise them to avoid the culprit food, give them an epinephrine injector, and refer them to an allergist for testing, education, and follow-up,” he advised.
Food allergies affect 6%-8% of pediatric patients, yet 35%- 90% of families self-report food allergies depending on the population studied. Milk, egg, wheat, peanuts, nuts, soy, and seafood account for 90% of food allergens. Most children outgrow allergy to milk, egg, wheat, and soy, while few outgrow allergy to peanuts, nuts, fish, and shellfish.
Most patients and many physicians believe that eczema is caused by food allergies. In fact, only a small minority of patients have food allergies that directly cause eczema. “Eczema could occur secondary to scratching induced by an urticarial food reaction or by a primary irritant reaction, but food directly causing isolated eczema is rare,” Dr. Bergman said. “The belief that food allergies directly cause eczema is completely understandable given that eczema patients do have an increased rate of allergies, the cyclic pattern of eczema, and the parent’s desire to find a cause for the child’s rash. Eczema’s cyclic nature can easily lead to a specific food being implicated due to recall bias. The parent will remember the flares that occurred with exposure to the specific food, while not recalling the times when the food was tolerated or the flares that were not associated with the food.”
If a parent is worried about a food causing eczema and there are no IgE mediated symptoms, then instead of testing he will often recommend that the family keep a formal food symptom diary while they are intermittently ingesting the food of concern. “The vast majority of parents will see no consistent direct correlation with the food and they can feel comfortable with ongoing future ingestion,” he said.
Some clinicians are offering oral immune therapy to patients with IgE-mediated food allergy. Dr. Bergman characterized such practice as “risky” at this point in time. “It’s like the traditional allergy shots you’d get for your pollen allergy, except it’s done orally,” he explained. “Research is being done in this area by introducing small amounts [of the allergen], in an attempt to induce tolerance. The results are encouraging, but the problem is that patients can have bad reactions. We also don’t know how well or for how long it will work. At this point, while promising, the field is not yet ready for prime time.”