Sputum eosinophilia has been shown to be a predictor of response to anti‐eosinophil therapies in patients with airway diseases. However, quantitative cell counts and differentials of sputum are labor intensive. The objective of this study was to validate a novel
ELISA‐based assay of eosinophil peroxidase ( EPX) in sputum as a rapid and reliable marker of airway eosinophils. Methods
The utility of
EPX‐based ELISAas an eosinophil‐specific assay was achieved through comparisons with sputum eosinophil differential counts in freshly prepared and archived patient samples from a variety of clinical settings. Results EPXlevels in sputum correlated with eosinophil percentage ( rs = 0.84) in asthma patients with varying degrees of airway eosinophilia. Significantly, unlike assays of other eosinophil granule proteins (e.g., ECPand EDN), which often detect the presence of these proteins even in asthma patients with neutrophilic bronchitis, EPX‐based ELISAlevels are not increased in this subset of asthma patients or in COPDpatients lacking evidence of an airway eosinophilia. Moreover, sputum EPXwas a surrogate marker of airway eosinophilia in other patient studies (e.g., allergen inhalation and treatment trials the anti‐( IL‐5) therapeutic Mepolizumab™). Finally, EPXlevels in cytocentrifuged prepared sputum supernatants correlated with those from rapidly prepared noncentrifuged filtrates of sputum ( rs = 0.94). Conclusion and clinical implication EPX‐based ELISAis a valid, reliable, repeatable, and specific surrogate marker of eosinophils and/or eosinophil degranulation in the sputum of respiratory patients. The novel EPX assay is a valid and reproducible eosinophil‐specific assay that can potentially be developed into a point‐of‐care assessment of eosinophil activity in airway secretions.