Potential for gradual tolerance acquisition in egg yolk-induced FPIES: A case series.

Background: Food protein-induced enterocolitis syndrome (FPIES) caused by egg yolk is increasingly recognized in Japan, yet the optimal timing for oral food challenge (OFC) and the process of tolerance acquisition remain unclear. Objective: Our aim was to investigate the clinical outcomes of early OFC (performed 90-120 days after the last symptomatic episode) in children with egg yolk-induced FPIES, with a focus on identifying a potential transitional tolerance phase. Methods: We conducted a retrospective analysis of a case series of 3 patients with egg yolk-induced FPIES who underwent early OFC. Subsequent ingestion outcomes and clinical symptoms were analyzed. Results: All 3 patients had negative OFC results but experienced mild gastrointestinal symptoms such as persistent diarrhea or recurrent vomiting during repeated ingestion at home. These features suggested a transitional tolerance phase. Full tolerance was achieved within 6 to 7 months in all cases. Conclusion: Early OFC may be conducted at a time that is suboptimal for reliable confirmation of tolerance. A negative OFC result does not necessarily indicate acquisition of full tolerance. Clinicians should be aware of a transitional phase during which symptoms may recur despite initial OFC success. Continued ingestion and close monitoring are essential to confirm full tolerance.