Adverse reaction to benzathine benzylpenicillin due to soy allergy: a case report

INTRODUCTION Soybean allergy is one of the most common food allergies especially among children. The Food Allergen Labeling and Consumer Protection Act (FALCPA) in the US requires the labeling of soy lecithin because it is derived from soybeans and may contain a number of IgE-binding proteins, possibly representing a source of hidden allergens. Here we describe a pediatric case of soy allergy misunderstood as drug allergy. CASE PRESENTATION An 11-year-old Caucasian girl was referred to our Allergy Unit because of the delayed appearance of an itching papular rash at the site of an injection of benzathine benzylpenicillin delivered by prefilled syringe. A skin test with benzathine benzylpenicillin and detection of serum-specific IgE to penicilloyl V, penicilloyl G, ampicillin and amoxicillin were negative. From her past medical history we know that, at the age of three years, she presented with edema of the lips and difficulty in breathing after eating a soy ice-cream. For that reason, she underwent a skin prick test with soybean that was negative and a serum-specific IgE to soybean test that was weakly positive (0.21KU/L). She underwent an oral provocation test with soy milk that yielded a positive result. CONCLUSIONS We describe a case of a patient with a delayed reaction to soy as a hidden allergen in a benzathine benzylpenicillin prefilled syringe. This case shows that lecithin contaminated by soy proteins and used as an excipient in drugs can cause reactions in patients with soy allergy. For that reason, the source of lecithin should always be specified among the constituents of drugs to avoid a source of hidden allergens and difficulties in the allergy work-up.