It is interesting to note that in our study there was discordance between sputum and blood eosinophils in about 40% of the patients with eosinophilic COPD at a peripheral blood eosinophil cut-off point of 0.3×109/L. This observation is similar to that of a recent study in patients with uncontrolled asthma where one-third of the participants exhibited discordance between blood and sputum eosinophils.35 In order to gain a better insight into our true positive (27 patients) and false negative (18 patients) cohorts, their demographic features, clinical characteristics and sputum cell counts were compared. The results nevertheless revealed no significant difference between the two groups (see Table S5 in the supplementary data). However, it should be highlighted that this analysis may have been underpowered due to small sample size. Plausible causes for the discordance between sputum and blood eosinophils may include the imbalance between the production and subsequent clearance of eosinophils by airway macrophages36 and variations in the process of recruitment of eosinophils into the airways.35 The fact that only a proportion of the eosinophilic COPD patients had blood eosinophilia may suggest the involvement of more than one distinct biological mechanisms underlying eosinophilic airway inflammation within this COPD phenotype.37 As in the case of asthma, Th2 cytokines could be responsible for inflammation in eosinophilic COPD.37 Nevertheless, eosinophilic airway inflammation in the absence of elevated levels of Th2 has also been reported in some COPD patients.38 One
It is interesting to note that in our study there was discordance between sputum and blood eosinophils in about 40% of the patients with eosinophilic COPD at a peripheral blood eosinophil cut-off point of 0.3×109/L. This observation is similar to that of a recent study in patients with uncontrolled asthma where one-third of the participants exhibited discordance between blood and sputum eosinophils.35 In order to gain a better insight into our true positive (27 patients) and false negative (18 patients) cohorts, their demographic features, clinical characteristics and sputum cell counts were compared. The results nevertheless revealed no significant difference between the two groups (see Table S5 in the supplementary data). However, it should be highlighted that this analysis may have been underpowered due to small sample size. Plausible causes for the discordance between sputum and blood eosinophils may include the imbalance between the production and subsequent clearance of eosinophils by airway macrophages36 and variations in the process of recruitment of eosinophils into the airways.35 The fact that only a proportion of the eosinophilic COPD patients had blood eosinophilia may suggest the involvement of more than one distinct biological mechanisms underlying eosinophilic airway inflammation within this COPD phenotype.37 As in the case of asthma, Th2 cytokines could be responsible for inflammation in eosinophilic COPD.37 Nevertheless, eosinophilic airway inflammation in the absence of elevated levels of Th2 has also been reported in some COPD patients.38 One
Peripheral_Blood_Eosinophils.docx