P. aeruginosa is more likely to be isolated from patients with more advanced disease and severely impaired lung function. It is, however, difficult to draw definitive conclusions regarding the extent to which the bacteria contributes to adverse clinical outcomes since severely reduced lung function by itself is a strong predictor of mortality in patients with COPD. Infection with P. aeruginosa might therefore be secondary to damaged lung tissue and decreased lung function, and thereby have no independent impact on the prognosis