Are the Australian guidelines asking too much of the Pneumonia Severity Index (PSI)?

To the Editor: The 2003 Australian guidelines on antibiotic therapy suggest that the Pneumonia Severity Index (PSI) may be used to triage site of care and antibiotic selection for patients with community-acquired pneumonia.1,2 The PSI was developed as a mortality prediction tool, using data from over 14 000 patients with community-acquired pneumonia.3 The antibiotic guidelines suggest specifically that PSI classes I and II represent patients suitable for outpatient therapy, and that class V can identify patients likely to require intensive care and broad-spectrum antibiotic therapy. We believe this is beyond the previously recommended applications of the PSI and advise caution about its use to identify patients with severe pneumonia.

In the cohort used to validate the PSI, only 32% of patients with severe pneumonia (requiring intensive care) were in class V, indicating that the PSI has poor sensitivity for severe pneumonia.3 This finding has been reflected in other studies.4 The strength of the PSI lies in its ability to identify low-risk patients, as the title of the validating article suggests.3 The PSI is so heavily weighted by age and comorbidities that younger patients needing intensive care are unlikely to accumulate enough points to reach class V. This is important, as early identification of patients with severe pneumonia and initiation of broad-spectrum antibiotic therapy and intensive-care support improves outcomes.

We are concerned that the PSI may be widely accepted for a purpose for which it was not intended and has not been validated. In underestimating the severity of illness in two-thirds of patients with “severe pneumonia”, the guidelines may provide false reassurance, while clinicians may lose confidence in the PSI if they find it “misses” most patients requiring intensive care.

Current evidence does support use of the PSI to guide decisions about inpatient or outpatient therapy. However, the modified British Thoracic Society (BTS) Severity Score is a simpler, better-validated tool to identify patients with “severe pneumonia” who are likely to need intensive care assessment5 (Box). This tool is useful for junior staff to “flag” patients with potentially severe pneumonia and ensure that they are discussed with a senior clinician. As always, the final management and antibiotic selection should be guided by clinical judgement. We believe that the antibiotic guidelines are valuable to encourage appropriate antibiotic use; our aim is to promote discussion of their content relating to this particular condition.