Associations between blood and sputum eosinophils and clinical features of mild-to-moderate COPD

Background: Blood eosinophilia could serve as a surrogate marker for airway eosinophilia and as a predictor of treatment response in severe COPD. In this study we aimed to assess the relationship between blood and induced sputum (IS) eosinophils and to investigate the association between blood and IS eosinophil number and clinical features of mild-to-moderate COPD. Methods: We performed a retrospective analysis of eosinophil counts in blood and IS, as well as demographic and lung function data in a cohort of stable, steroid-naïve (GOLD 1 or 2) COPD patients and control subjects. Results: Ninety COPD patients (48% women, median age 64.5 (IQR 58-72) years and median post-bronchodilator FEV1 70 (IQR 59-80) % of predicted) and twenty controls were included. Median blood eosinophil count was 180 (IQR 90-270) cells/µl and this number was used as a threshold value to classify patients as having high (≥180 cells/µl) or low (<180 cells/µl) blood eosinophil number. The median IS eosinophil percentage was 1 (IQR 0-9)% and 3% was used as a threshold value for high (≥3%) or low (<3%) sputum eosinophil percentage. There was no correlation between blood and IS eosinophil levels in COPD (r=-0.04 p=0.705). We found no difference in demographic data and pulmonary function between COPD patients with high and low blood or sputum eosinophil number. There was weak, albeit significant correlation (r=0.22 p=0.041) between blood eosinophil level and age in COPD patients. Conclusions: Blood eosinophilia poorly relates to airway eosinophilia in stable mild-to-moderate COPD patients. Our data suggest that blood eosinophil number in COPD patients might increase with age.