Inclusion Criteria:  All the patients met the diagnosis of COPD according to Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines and had definite airflow limitation with a post-bronchodilator forced expiratory volume in 1 second (FEV1) / forced vital capacity (FVC)<0.7. They were admitted to the ward of Department of Respiratory and Critical Care Medicine due to COPD exacerbation.  Exclusion Criteria:  age <40 years; subjects with airway diseases other than COPD; comunity acquired pneumonia; active tuberculosis; severe liver or renal dysfunction; malignancy; HIV infection or immunodeficiency; ever received glucocorticoid in the past month.