NEW YORK – Children with atopic dermatitis have a high risk of food allergies – although these allergies are not always easy to pinpoint.
"In this population, up to 20% of [those] with mild to moderate atopic dermatitis and 30%-40% of the severe patients will have a true food allergy," that can be confirmed with an open food challenge. Dr. Lawrence F. Eichenfield said at the American Academy of Dermatology’s 2010 meeting.
However, he said, positive skin prick testing – a common form of allergen identification – isn’t a very accurate way to detect the allergies. As a result, parents of children with atopic dermatitis (AD) will frequently state that their child has a food allergy, when, in fact, none exists.
A new national guideline helps clarify that issue, he said. Published in late 2010, "Guidelines for the Diagnosis and Management of Food Allergy in the United States," from the National Institute of Allergy and Infectious Diseases, provides some helpful information for dermatologists trying to make the AD/food allergy connection.
The document defines a food allergy as an adverse health effect arising from a specific immune response that occurs reproducibly on exposure to a given food. Skin prick testing, however, can only identify sensitization to a food, which doesn’t necessarily correlate with a clinical event, said Dr. Eichenfield, a professor of clinical pediatrics and dermatology at the University of California, San Diego.
"It’s very clearly written in the guidelines that an individual can develop allergic sensitization without having clinical symptoms on exposure to the food. Therefore, skin testing is not sufficient to say there is a food allergy."
Nevertheless, a common clinical scenario in the pediatric dermatology office is a parent who claims the child is allergic to a given food – most often eggs, soy, milk, wheat, or peanuts – because of a positive skin test. "What people don’t understand is, these tests are neither very sensitive nor specific," he said.
The specificity of skin prick testing hovers at about 85%, while the sensitivity is around 75%. "That means if we assume a 5% true milk allergy in a group of 1,000 people, skin prick testing will identify 42 of the 50 with a true allergy, miss 8 of those with the allergy, and give a false positive result to 238 people without the allergy," he said. "It’s a problem when you start labeling someone as having an allergy with just a positive test, but no clinical indicator."
The national guidelines stress that both family history and AD are risk factors for food allergy. The report suggests that children younger than 5 years who have moderate to severe AD that is uncontrolled despite optimal treatment, should be tested for allergies to milk, egg, peanut, soy and wheat. A positive, reliable history of a clinical reaction immediately after exposure to a specific food is also grounds for an investigation, according to the report.
Oral food challenge is probably the best way to determine a true food allergy, but can only be carried out in an environment set up to cope with severe reactions – usually an allergist’ s office. The good news is that the common overrepresentation of "food allergies" among children with AD means that many can safely consume foods that have been, literally, taken off the table.
A 2010 retrospective study looked at 125 children with atopic dermatitis, 44 of whom had suspected allergies to a variety of foods. All of them underwent oral food challenges. At baseline, there were 111 reports of food avoidance due to positive skin prick testing – foods included egg, fruit, meat, vegetables, milk, soy, wheat, shellfish, oats, and peanuts (J. Pediatr. 2010;158:578-83).
"Except for wheat, 80% or more of the oral food challenges were negative to the foods being avoided," wrote Dr. David M. Fleisher of the University of Colorado, Denver, and colleagues. This study was funded by National Jewish Health and the authors declared no conflicts. The only positive reactions were for egg, banana, peanut, soy, and wheat.
"Depending on the reason for avoidance, 84%-93% of the foods being avoided were successfully reintroduced into the diet," Dr. Eichenfield said.
Dr. Lawrence F. Eichenfield
The new management guidelines have affected the way he assesses AD patients, Dr. Eichenfield added. "Now I ask about food allergy reactions. If I find anything positive, I take a very detailed history, including the type of reaction, the time course, and the consistency of it after exposure to the food. Many times, just by asking, you will uncover a very significant reaction, like contact dermatitis, urticaria, or even anaphylaxis."