Predicting bacteraemia or rapid identification of the causative pathogen in community acquired pneumonia: where should the priority lie?

Community-acquired pneumonia (CAP) remains amongst the most common causes of infectious disease-related death worldwide. However, despite its clinical importance the existing routine microbiology tests for CAP pathogens have significant limitations, lacking sensitivity for identifying the causative pathogen and only altering management in a minority of patients. For example, blood cultures identify a pathogen in less than or equal to 10% of CAP cases [1, 2] and results are usually only available after 24h. Therefore, clinicians are still required to prescribe broad-spectrum antibiotics for the first 24-72h, which is the period of highest risk for clinical deterioration and death [3, 4]; even when bacteria are cultured this only rarely leads to a change in treatment and may miss co-infection [2, 5]. These limitations have led to suggestions that patients admitted with CAP do not require routine microbiological testing with management decisions based solely on clinical factors. However, not identifying the causative pathogens in CAP has important implications. From a public health perspective, not performing microbiological testing could result in failure to identify important changes in microbial aetiology or changes in anti-microbial resistance patterns. In the absence of microbiological testing all patients would be treated with prolonged broad-spectrum antibiotics that may not be required if, for example, Streptococcus pneumoniae was the causative pathogen, thereby unnecessarily increasing drug cost and potentially promoting the development of antimicrobial resistance or anti-microbial-related complications such as Clostridium difficile diarrhoea [6]. Finally, the relatively rare CAP patient infected with a drug-resistant pathogen, such as community-acquired methicillin-resistant Staphylococcus aureus or Pseudomonas aeruginosa, may not be identified, thus increasing the chance of a poor outcome.