Failure of first line agents:

• no improvement in symptoms after completion of antibiotic therapy or
• clinical deterioration after 72 h of antibiotic therapy.

Risk factors for treatment failure:

- suboptimal adjunctive nonantimicrobial therapy

- FEV₁ < 50% predicted

- chronic oral corticosteroid use

- home oxygen therapy

- significant comorbidities, e.g. cardiac disease, debilitation, malnutrition.