False positives common in siblings of food-allergic children

Parents of a child with a food allergy are often concerned that their other children are at high risk for allergies as well, and oftentimes take them to the doctor to be screened with a blood test or skin prick. A new study, however, is showing that this may not be necessary as siblings have only a slightly higher risk than the general population, about 13% compared with 8%, of having a food allergy.

These findings were presented at the American College of Allergy, Asthma & Immunology annual meeting last week and also showed that more than 50% of the same siblings tested positive for a food allergy although they showed no signs of an actual food allergy. With that in mind, the authors of the study said, parents should limit testing siblings with blood and skin prick tests.

“A large proportion of these parents are unnecessarily avoiding a food their child could eat,” Ruchi Gupta, lead author on the study and an associate professor of pediatrics at Northwestern University Feinberg School of Medicine, said in the article.

False positives cause unnecessary stress and fear for parents and children, and, somewhat paradoxically, could increase the risk of a real allergy, the article states. For example, avoiding a potential allergen may increase the risk of developing food allergies later in life as was the case in a major study earlier this year which found that exposing infants to peanuts before the age of one significantly lowered their risk of developing an allergy to the nuts in the future.

“Testing for food allergies if a reaction hasn’t taken place can provide false-positives, as we saw in our research,” Gupta said. “More than half the kids in the study had a sensitivity to a food, but they weren’t truly allergic. Kids who have a food sensitivity shouldn’t be labeled as having a food allergy.”

For the sibling study, Gupta and colleagues tested 1,120 children, from birth to age 21, with blood tests and skin pricks, then assessed true food allergy based on those tests plus the child’s history of reacting to the food. Among siblings of children with a food allergy, 33% tested negative for an allergy.

Of the remaining two-thirds who tested positive, only 13% had a true food allergy, most commonly to milk or egg. The remaining 53% of the siblings tested positive yet had no actual allergy. As stated in the article, those results are in accord with a 2013 Australian study that found having an immediate family member with a food allergy only modestly increased an infant’s allergy risk.

“This perceived risk is a common reason to seek ‘screening’ before introducing a high-risk allergen to siblings. But screening a child before introducing a high-risk allergen isn’t recommended,” study co-author and allergist Dr. Matthew Greenhawt said in an article. “Food allergy tests perform poorly in terms of being able to predict future risk in someone who has never eaten the food before.”

However, if parents are worried about allergies, Gupta recommends discussing concerns with an allergist and potentially following up any positive blood test or skin pricks with a food challenge that can be performed safely in their office.

“You can try it in a safe environment and make sure the child does or does not [have an allergy], so they can keep the food in their diet if possible,” she added.

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