Abstract： Objective:To study the correlation between peripheral blood eosinophils levels and clinical characteristics of patients with acute exacerbation of chronic obstructive pulmonary disease (AECOPD).Methods:A retrospective study was conducted in 79 AECOPD patients admitted to the Department of Respiratory and Critical Care Medicine of Peking University Third Hospital from January 2015 to December 2016. Peripheral blood EOS level 100/μl, 300/μl, 2%, 3%, and sputum EOS level 3% were the cut-off values. Patients were divided into EOS≥100/μl group ( n=33), EOS<100/μl group ( n=46); EOS≥300/μl group ( n=10), EOS<300/μl group ( n=69); EOS%≥2% group ( n=27), EOS<2% group ( n=52); blood EOS≥3% group ( n=16), blood EOS<3% group ( n=63); sputum EOS≥3% group ( n=7), sputum EOS<3% group ( n=15) respectively. The study analyzed the differences of clinical features between each two groups of patients; According to the blood EOS level in the acute phase and the recovery phase, ≥100/μl was high and<100/μl was low, they were divided into high EOS in both acute and recovery phase group ( n=21), high EOS in the acute phase and low EOS in the recovery phase group ( n=4), low EOS in the acute phase and high EOS in the recovery phase group ( n=20) and low EOS in both acute and recovery phase group ( n=12). The differences of clinical characteristics in each group were analyzed. Results:The median duration of COPD, proportion of patients with forced expiratory volume in the first second (FEV 1) percentage of predicted value (FEV 1% predicted value)<50%, proportion of complicated with respiratory failure, median level of procalcitonin, proportion of systemic glucocorticoid therapy and median length of stay in the EOS≥100/μl group were all significantly lower than EOS<100/μl group (5 vs 13 years, 48.0% vs 81.8%, 21.9% vs 50.0%, 0.04 vs 0.09 μg/L, 21.2% vs 56.5%, 11 vs 14 d, all P<0.05). The proportion of re-acute exacerbation in the EOS≥300/μl group was significantly higher than EOS<300/μl group (60.0% vs 23.2%), while the median fibrinogen level and median procalcitonin level were significantly lower than EOS<300/μl group (3.38 vs 3.85 g/L, 0.03 vs 0.07 μg/L, all P<0.05). The median duration of COPD, proportion of FEV 1% predicted value<50%, proportion of complicated with respiratory failure, the modified British Medical Research Council′s dyspnea scale (mMRC) score, median fibrinogen level, median C-reactive protein level, median procalcitonin level, proportion of systemic glucocorticoid therapy, proportion of non-invasive ventilation during hospitalization and median length of stay in the EOS≥2% group were all significantly lower than EOS<2% group [5 vs 13 years, 40.9% vs 83.3%, 12.0% vs 51.0%, 3 (2, 3) vs 3 (3, 4) points, 3.37 vs 3.97 g/L, 3.6 vs 16.8 mg/L, 0.04 vs 0.09 μg/L, 14.8% vs 55.8%, 0 vs 19.2%, 9 vs 14 d] (all P<0.05). The median duration of COPD, proportion of complicated with respiratory failure, median level of C-reactive protein and proportion of systemic glucocorticoid therapy in the blood EOS≥3% group were significantly lower than blood EOS<3% group (5 vs 10 years, 6.7% vs 45.8%, 4.4 vs 12.9 mg/L, 12.5% vs 49.2%) (all P<0.05). The median duration of COPD and median length of stay in the sputum EOS≥3% group were significantly shorter than sputum EOS<3% group (2 vs 15 years, 10 vs 21 d), while the median blood EOS count and median blood EOS ratio were significantly higher than sputum EOS<3% group (0.20 vs 0.01×10 9/L, 2.4% vs 0.1%) (all P<0.05). The proportion of complicated with respiratory failure and received systemic glucocorticoid therapy in the high EOS in both acute and recovery phase group were significantly lower than the low EOS in the acute phase and high EOS in the recovery phase group (14.3% vs 75.0%, 14.3% vs 55.0%) (all P<0.05). The proportion of FEV 1% predicted value <50% in the high EOS in the acute phase and low EOS in the recovery phase group was significantly lower than the low EOS in the acute phase and high EOS in the recovery phase group (0 vs 82.4%) ( P<0.05). The median FEV1% predicted value level in the high EOS in the acute phase and low EOS in the recovery phase group was significantly higher than the low EOS in both acute and recovery phase group (59.5% vs 36.0%) ( P<0.05). Conclusions:High EOS AECOPD patients have a shorter duration of disease and fewer days of hospitalization. The proportion of patients complicated with respiratory failure is lower. It is easier to distinguish the differences in clinical characteristics of AECOPD patients with a blood EOS count of 100/μl as a cut-off value. EOS levels in the acute phase and the recovery phase are helpful to distinguish the clinical characteristics of AECOPD patients.