AbstractBackgroundPractical biomarkers for endotypic characterization of chronic rhinosinusitis (CRS) remain elusive, hindering clinical utility. Eosinophil peroxidase (EPX) is an enzyme released by activated eosinophils. The objective of this study was to evaluate a clinic EPX assay as a marker of eosinophilic CRS.MethodsSubjects with and without CRS presenting to a tertiary care rhinology clinic were prospectively enrolled, and nasal cytology brushings were collected from the middle meatus during in‐clinic nasal endoscopy. ELISA assay was used to quantify EPX levels, and a customized multiplex immunoassay was used to quantify inflammatory cytokine mediators. Findings were correlated with clinical data.ResultsForty‐two subjects were enrolled, including 31 CRS subjects and 11 controls. Median EPX levels were 125.0 ng/mL (standard deviation [SD] 1745.8) and 6.5 ng/mL (SD 99.0) for CRS group and controls, respectively (p = 0.003). EPX levels were associated with history of asthma (p = 0.015), allergies (p = 0.028), polyps (p = 0.0006), smell loss (p = 0.006), and systemic eosinophilia or elevated immunoglobulin E (p ≤ 0.0001). Twenty‐eight subjects from both the CRS and control groups had prior pathology for comparison, with histologic confirmation of local tissue eosinophilia (>10 eosinophils/hpf) in 11 subjects. This subgroup had a median EPX level of 967.5 ng/mL compared to 10.6 ng/mL in 17 subjects without local tissue eosinophilia (p = 0.0008). EPX levels were positively correlated to interleukin‐5 levels (p = 0.0005).ConclusionEPX levels can be measured via well‐tolerated in‐clinic collection of nasal mucus. EPX levels are associated with clinical markers of type 2 inflammation and tissue eosinophilia and may provide a valuable diagnostic tool to delineate eosinophilic CRS.