For COPD patients, patients who are admitted to the Prince of Wales Hospital with AECOPD will be screened for this study. AECOPD is defined when a patient with background COPD (with lung function parameters of FEV1/FVC ratio <70%)24 presented with at least two of the following major symptoms (increased dyspnoea, increased sputum purulence, increased sputum volume) or one major and one minor symptom (nasal discharge/congestion, wheeze, sore throat, cough) for at least two consecutive days.25,26