IntroductionCell count in induced sputum is a noninvasive biomarker to assess airway inflammation phenotypes. Accordingly, sputum cell counts are extensively used in the treatment of asthma and COPD. Nevertheless, the clinical application of sputum cell counts in patients with asthma–COPD overlap (ACO) remains elusive. The aim of this study was to investigate sputum cell counts in patients with ACO which are different from those in patients with asthma and COPD and also to examine the relationship between sputum cell counts in bronchial reversibility and bronchial hyperresponsiveness (BHR).Patients and methodsA total of 374 patients participated in the study, including 142 patients with asthma, 160 patients with COPD and 72 patients with ACO. All patients underwent the following tests on the same day: pulmonary function test (PFT), BHR test or bronchodilator reversibility test and inducing sputum. They were classified into the asthma group, COPD group or ACO group based on a clinical history, PFT values and BHR test or bronchodilator reversibility test.ResultsThe three groups had different PFT values (p<0.001) except for forced vital capacity (FVC) between the asthma and ACO groups (p=0.378). The sputum levels of eosinophil% were decreased in patients with COPD when compared with those in patients with asthma and ACO (p<0.001 and p<0.001, respectively). There was a difference in sputum neutrophil% and macrophage% counts among the three groups (p<0.001 and p<0.001, respectively); there was no difference in sputum eosinophil% counts between patients with ACO and asthma (p=0.668) and there was no difference in the percentage of induced sputum cells between the stage of airway obstruction and the stage of BHR.ConclusionThe clinical relevance of this study provides evidence that sputum cell counts as an inflammatory biomarker could carry some information to distinguish ACO, asthma and COPD, and these biomarkers need more studies to provide diagnostic value in the differentiation between ACO, asthma and COPD.