Predictors of sputum eosinophilia in severe asthma

Background: In asthmatic patients, airway eosinophilia identifies a phenotype of patients that suffer from recurrent exacerbations and respond to biologic therapies. Direct measurement, through sputum induction, is impractical as part of routine clinical care so blood eosinophil counts are commonly used as a surrogate marker. We sought to assess the degree of concordance between these measures in our cohort of severe asthma patients. Methods: WATCH patients with severe asthma had blood tests, FeNO measurement and sputum induction performed on the same day. Spearman’s rank correlation coefficients and classification tables were calculated between sputum eosinophil, blood eosinophil, serum IgE and FeNO measures. Prediction of sputum eosinophilia (>3%) was assessed using receiver operating curves. Results: 203 patients, treated with high dose inhaled corticosteroids, were included in the analysis. Using commonly used cut-offs for blood eosinophilia (>300 cells/μL) and sputum eosinophilia (>3%), measurements were concordant in 138 patient (73.0%). 22 patients (11.6%) demonstrated blood eosinophilia alone and 29 patients (15.3%) sputum eosinophilia alone. Conclusions: We find that in a proportion of patients with severe asthma have sputum eosinophilia not recognised by blood. A better understanding of the nature of these airway eosinophils and the basis for the lack of apparent systemic IL5 signalling would help understand if these are a potentially biologically overlooked population or represent a different disease endotype.