Antibiotics are commonly prescribed empirically for lower respiratory infections. Infections of the airway mucosa are much more common than pneumonia and the illness they cause is less severe because the infection is superficial, most of the bacteria being found associated with mucus in the lumen. In many cases the infection will resolve spontaneously without antibiotic treatment. Most adult patients are experiencing an exacerbation of chronic lung disease, particularly chronic obstructive pulmonary disease (COPD), when neutrophilic inflammation in response to bacterial infection leads to increased sputum volume and viscosity, and breathlessness due to airflow obstruction. In these circumstances, bacteria are cultured from sputum in about half of the cases which means that, in some of the others, accepting that sputum culture is not a sensitive investigation, antibiotics are given unnecessarily. Antibiotics are essential when a patient with severe COPD presents with purulent sputum and systemic symptoms of infection, but they are often given either to speed up recovery from a bacterial infection that might be expected to resolve spontaneously following a successful host inflammatory response, or in a defensive manner to avoid the risk of airway infection progressing to pneumonia and causing deterioration in a more compromised patient whose host defences are more seriously impaired.

In recent years attention has rightly focused on trying to define which patients benefit from antibiotic treatment, and those in whom antibiotics can be avoided. The size of …