Eosinophil Peroxidase as a Dynamic Indicator of Treatment Response in Eosinophilic Chronic Rhinosinusitis

Background Eosinophil peroxidase (EPX) in nasal mucus correlates with markers of type 2 inflammation and tissue eosinophilia in chronic rhinosinusitis (CRS). This study evaluated EPX as a dynamic biomarker across treatment modalities and its potential prognostic value in eosinophilic CRS. Methods Patients with bilateral CRS undergoing treatment with endoscopic sinus surgery (ESS) or dupilumab injections, as well as non‐CRS controls undergoing endoscopic endonasal pituitary surgery were prospectively enrolled. Nasal cytology brushings from the middle meatus were collected pre‐and post‐treatment, except in the dupilumab group (sampled once after stabilization on medication). EPX was quantified via ELISA and correlated with clinical data. Results Twenty‐nine CRS patients and 7 non‐CRS controls were included. EPX levels decreased post‐treatment in both treatment groups. Median paired change was −233.5 ng/µl (CRS ESS group, p = 0.015) and 0 ng/µl (non‐CRS control group, p = 0.57). The difference of medians (non‐paired) in the dupilumab‐treated CRS group as compared to the pre‐treatment CRS group was −460.9 ng/µl ( p = 0.027). A strong negative correlation was observed between pre‐treatment EPX concentration and ΔSNOT‐22 score (Spearman's p = 0.70, p = 0.038), indicating that higher pre‐treatment EPX levels were associated with greater improvements in SNOT‐22 scores. Conclusion EPX levels are elevated in CRS and decline with treatment. Higher baseline EPX may predict greater post‐treatment symptom improvement, supporting EPX as a marker of disease activity and treatment response in CRS. Summary Eosinophil peroxidase (EPX) correlates with local type 2 inflammation and may serve as a noninvasive biomarker of eosinophilic chronic rhinosinusitis. EPX declines after surgery and dupilumab, highlighting its potential to track treatment response. High baseline EPX predicts patients likely to achieve a clinically meaningful Sinonasal Outcome Test‐22 change.