SAN FRANCISCO – A 9-mm wheal after skin prick testing provided a 95% positive predictive value for egg or peanut allergy in an analysis of data from 5,000 12-month-old infants recruited from the general population.

The current clinical practice of diagnosing peanut or egg allergy in infants who develop wheals larger than 8 mm from skin prick testing is appropriate in the general population, Lyle Gurrin, Ph.D., and his associates reported at the annual meeting of the American Academy of Allergy, Asthma, and Immunology.

Infants with a wheal of any size after a skin prick test were invited for a diagnostic oral food challenge unless they had a convincing reaction in the previous month.

The analysis included 181 infants who underwent peanut challenges, 310 with egg challenges, and 71 with sesame challenges. None of the wheal sizes after sesame challenges reached a 95% positive predictive value for allergy, said Dr. Gurrin of the University of Melbourne.

Previous studies have suggested skin-prick-test wheal sizes provide a 95% likelihood of food allergy if the wheal is 8 mm or larger for egg allergy and 7 mm or larger for peanut allergy. Most of those were small studies of high-risk patients drawn from clinics, not the general population. The studies included a broad range of ages and relied on a history of ingestion reaction rather than performing a formal food challenge.

The current analysis used data from the larger HealthNuts study, a population-based study of Australian 1-year-old infants that was conducted to identify the prevalence of food allergy and modifiable risk factors. Investigators recruited parents and infants at childhood immunization sessions, and 2,848 (73%) agreed to participate. Skin prick tests showed sensitization (a wheal of 1 mm or larger) to peanut in 9%, to raw egg white in 16%, and to sesame in 3% (J. Allergy Clin. Immunol. 2011;127:668-676.e2).

Oral food challenges in these sensitized patients proved that 3% of the entire cohort was allergic to peanut, 9% was allergic to raw egg 1%, and had sesame allergy. Of the infants with raw egg allergy, 80% were able to tolerate baked egg. Some infants were allergic to more than one food. Overall, more than 10% of the cohort had challenge-proven IgE-mediated allergy to one of the common allergenic foods of childhood.

Further analysis of wheal-size thresholds for diagnosis will stratify the findings by the presence or absence of eczema, a family history of allergy, and the ingestion/reaction history, Dr. Gurrin said.

A positive oral food challenge was defined as three or more concurrent, noncontact urticaria lasting at least 5 minutes, vomiting, periorbital angioedema, or anaphylaxis occurring within 2 hours of ingesting the test food.

No patients were excluded from the HealthNuts study due to severe eczema, said one of Dr. Gurrin’s study associates, Dr. Katrina Allen, also of the university.

Less than 1% of infants in the study were dark skinned, but demographic factors did not differ significantly between infants who were excluded from the study and those who were enrolled, she said during the question-and-answer session after Dr. Gurrin's presentation.