Abstract
Assessment of the significance of the presence or absence of pathogenic bacteria in sputum specimens is made difficult by a number of factors which are discussed. It is now generally accepted that Haemophilus influenzae is the organism most commonly responsible for recurrent or persistent sputum purulence in chronic lower respiratory disease. The close association between this organism and chronic bronchitis has been confirmed by studies of precipitin antibodies. In patients with cystic fibrosis precipitin antibodies to Pseudomonas aeruginosa and Staphylococcus aureus are frequently present: they are less commonly found in patients with bronchiectasis and very rarely in patients with chronic bronchitis.