Date Published: July 11, 2012
Publisher: BioMed Central
Author(s): Zein Faraj, Harold L Kim.
Cow’s milk and hen’s egg are the most frequently encountered food allergens in the pediatric population. Skin prick testing (SPT) with commercial extracts followed by an oral food challenge (OFC) are routinely performed in the diagnostic investigation of these children. Recent evidence suggests that milk-allergic and/or egg-allergic individuals can often tolerate extensively heated (EH) forms of these foods. This study evaluated the predictive value of a negative SPT with EH milk or egg in determining whether a child would tolerate an OFC to the EH food product.
Charts from a single allergy clinic were reviewed for any patient with a negative SPT to EH milk or egg, prepared in the form of a muffin. Data collected included age, sex, symptoms of food allergy, co-morbidities and the success of the OFC to the muffin.
Fifty-eight patients had negative SPTs to the EH milk or egg in a muffin and underwent OFC to the appropriate EH food in the outpatient clinic. Fifty-five of these patients tolerated the OFC. The negative predictive value for the SPT with the EH food product was 94.8%.
SPT with EH milk or egg products was predictive of a successful OFC to the same food. Larger prospective studies are required to substantiate these findings.
Although estimates of prevalence are heterogeneous in medical literature, cow’s milk and hen’s egg are consistently reported as two of the most common food allergens in the pediatric population . The diagnostic investigation for food allergy commences with skin prick testing (SPT) with commercial extracts of suspected allergens. In cases of true IgE-mediated allergic reactions, a localized cutaneous swelling in the form of a ‘wheal’ usually ensues. Typically, negative SPTs are followed by an oral food challenge (OFC), the gold standard, to definitively rule out food allergy.
A retrospective chart review was performed on all patients undergoing cow’s milk or hen’s egg skin prick testing at a single allergy and immunology clinic in Kitchener, Ontario during a 2 year time period from 2009–2011. Patients were deemed eligible if they were between the ages of 6 months and 18 years at the time of an initial positive SPT to cow’s milk and/or hen’s egg commercial extracts, had a subsequent negative SPT to the EH version of the allergen, and proceeded to an OFC with the EH milk or egg product. An SPT was considered positive if the wheal’s diameter was at least three millimeters larger than the negative control test. All eligible subjects had either previously experienced an allergic reaction to milk or egg or had a positive SPT predictive of an allergy. Subjects with a history of reaction to baked milk or egg products were excluded from the study.
Of the 128 subjects found to have a positive SPT to unheated cow’s milk or hen’s egg, 58 subjects (median age 3.5 years; range 1.25–13 years) met the remainder of the inclusion criteria for this study. Fourteen were milk-allergic whereas 40 were egg-allergic (Figures 1 and 2). The median age at first reaction was 1 year (range 0.5–7 years). Initial allergic reaction varied but included cutaneous symptoms such as hives, pruritis or flushing (84.5%), upper airway symptoms including sneezing or throat symptoms (10.3%), lower airway symptoms including wheeze-bronchospasm or respiratory distress (3.5%), gastrointestinal symptoms such as nausea, vomiting, cramping abdominal pain, bloating, diarrhea (22.4%), and cardiovascular symptoms evidenced by dizziness or hypotension (3.5%). Anaphylaxis was reported in 8.6% of patients at initial reaction (Table 1).
The vast majority of subjects with a negative fresh food SPT of the EH food product had successful OFCs to the baked muffins. Fifty-five of the 58 patients tolerated the oral challenge and were encouraged to re-introduce baked eggs or milk into their diets, whereas three reacted and were assumed to be allergic to the food products in all forms. Only one of the three had an anaphylactic reaction (Figure 3). The negative predictive value for the SPT with the extensively heated food product was 94.8%.
The majority of patients with milk or egg allergy who had negative SPTs with the EH milk or egg products respectively tolerated the EH form of the food in an OFC. Based on the data collected in this study, skin prick testing with EH food carries a negative predictive value of 94.8% and may be a reliable marker for identifying children likely to tolerate EH milk or egg.
SPT: Skin prick test; OFC: Oral food challenge; EH: Extensively heated.
The authors declare that they have no competing interests.
ZF helped design the study, conducted the chart review for data collection, analyzed the data and wrote the paper. HK conceived the study and was the primary clinician performing all tests and challenges on subjects. Both authors read and approved the final manuscript.
Zein Faraj is a senior medical student at McMaster University, an MD Candidate in the class of 2012. She has previously completed Bachelor of Health Sciences (Honours) degree at McMaster University.