March 23, 2011

By Fran Lowry

NEW YORK (Reuters Health) Mar 21 - A large wheal from a skin prick test (SPT) is a reliable predictor of true food allergy among the general population, according to new research presented this weekend in San Francisco at the 2011 Annual Meeting of the American Academy of Allergy, Asthma and Immunology (AAAAI).

"Skin prick tests are easy to administer and are done in specialized allergy clinics, but the problem is they have low predictability for true food allergy for those who have never eaten the food nor had an allergic reaction," Dr. Lyle Gurrin, from the Melbourne School of Population Health and Murdoch Children's Research Institute in Melbourne, Australia, told Reuters Health. "Our study is the first population-based study to assess the use of SPT predictors at the general community level."

Dr. Gurrin and his colleagues wanted to see whether there was a point on the SPT scale where the wheal size would be so large that it could be taken to mean that the subject did in fact have a food allergy and therefore would not need to have a resource-intensive and potentially dangerous food challenge test.

Such thresholds have been developed but only in subjects already known to be at high risk for food allergy, not in the general population, he told Reuters Health.

The study included 5,000 twelve-month-old infants from community immunization clinics in Melbourne who were all given skin prick tests to raw egg white, peanut and sesame. One thousand children had a reaction to the skin test > 5.1 mm and were referred to the Royal Children's Hospital for a food challenge test, considered the gold standard for determination of food allergy.

The researchers found that in infants not previously exposed to the food, the larger the wheal, the more likely the child was to have a food allergy.

Specifically, they said in their abstract for the meeting, wheal sizes linked with a 95% positive predictive value were 5 mm for raw egg white allergy (95% CI 4 mm-6 mm), 10 mm for baked egg allergy (95% CI 9 mm-11 mm), 9 mm for peanut allergy (95% CI 8 mm-10 mm), and 8 mm for sesame allergy (95% CI 7 mm-9 mm).

"We are trying to help clinicians be more confident that those children with large wheal sizes who have never eaten the food are highly likely to be food allergic and therefore may be able to avoid unnecessary food challenges to confirm this," Dr. Gurrin said.

Children with smaller wheal sizes are sensitized to the allergen, but this does not mean that they are truly allergic in the absence of a history of a clinical reaction, he emphasized.

"We are very concerned that in these times, when food allergy is getting a lot of press, that people will take their children with SPT wheal size 4 mm to egg and peanut and who have never eaten the food are declared allergic and start avoiding peanuts and egg and all sorts of other healthy foods, when in fact they really are not allergic," Dr. Gurrin said.

"The bottom line is, with a sufficiently high threshold, kids can avoid potentially dangerous food challenge testing."


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