Introduction: Patients with food allergies need personalized information on their risk of reaction in "real-life" situations. This multicentric study aimed to investigate the link during accidental reactions between the nature and amount of food allergens consumed in "real-life situation" and the severity of the symptoms.

Methods: Patients were prospectively recruited from December 1, 2020, to December 31, 2021, at the emergency departments in the Geneva University Hospitals and local pediatric emergency facilities, through an allergy outpatient clinic, at school and daycare facilities and trough their primary care physicians. Medical history of patients presenting reactions suggestive with immediate food allergy and suspected food samples were collected. Allergy diagnostic tests were retrospectively and prospectively collected. The samples were analyzed for their allergen content.

Results: We recruited 147 subjects with an accidental immediate-type allergic reaction to a food. We were able to collect 115 reaction-eliciting food samples allowing to quantify the allergen amount causing the reaction, as well as correlating this amount to the severity of the reaction. Children represented a large part of the reactors, and most reactions were to common food allergens such as tree nuts, cow's milk as well as peanuts and hen's egg. Reactions were mostly to prepackaged foods and seven reactions were to products with precautionary allergy labeling, or without labelling to the eliciting allergen. Reactions were of various degrees of severity, and independent to the amount of allergen ingested.

Discussion: The severity of reactions did not show a direct correlation with allergen quantity, emphasizing individual sensitivity. Some reactions occurred with allergen amounts significantly below the legal limit for mandatory labelling of 1 g/kg (1000 ppm) in Switzerland. The study also highlighted considerable variability in allergen concentrations in foods labeled with possible "contaminations" or "traces". These findings raise questions about the accuracy of allergen labeling and regulations in Switzerland.