Microbiological diagnostic tests for community-acquired pneumonia are useful

To the Editor: Influenza causes around 8% of community-acquired pneumonia (CAP) episodes,1 and during the (H1N1) 2009 influenza pandemic, concurrent bacterial infections were detected in up to 29% of fatal infections.2 Determining microbial aetiology of CAP can guide antibiotic and antiviral therapy.

To investigate the usefulness of microbiological diagnostic testing for CAP, we undertook a retrospective review of our pathology department’s electronic database of patients admitted with CAP to a tertiary referral centre (365 beds) and two suburban teaching hospitals (314 and 280 beds) between 1 May 2007 and 30 April 2008. Inclusion criteria were: International Classification of Diseases, 10th revision codes J09–J22 (respiratory tract infections); age ≥ 18 years; and consolidation on a chest radiograph performed within 48 hours of admission. Exclusion criteria were: hospitalisation within the previous 14 days; admission to a unit managing predominantly immunosuppressed patients; HIV infection; active tuberculosis; chest injuries; and previous inclusion in the study. Basic demographic data and microbiological investigation results were collected. We adopted the Australian CAP Study’s criteria for aetiology and classification of good-quality sputum.1 Change in management was indicated for patients with bacteria not covered by empiric regimens recommended in Therapeutic guidelines: antibiotic, version 13, or organisms with public health and infection control implications.3 Continuous variables were analysed using either Student’s t test or the Wilcoxon rank-sum test, and categorical variables with the Fisher exact test. Statistical significance was set at P < 0.05.

From 2436 admissions, 341 patients met inclusion criteria. Mean age was 71.1 (SD, 17.1) years, and 72 patients (21.1%) were in residential care. Most patients (251; 73.6%) had investigations to determine aetiology (Box). Ninety-three organisms were identified from 83 patients (33.1%), most commonly Streptococcus pneumoniae (33; 13.1%), Haemophilus influenzae (14; 5.6%), influenza (11; 4.4%) and Legionella spp (9; 3.6%). Good-quality sputum taken within 8 hours of presentation had the highest diagnostic yield (47.1%). Changes to management were indicated for 37 patients (14.7%).

Aetiology was identified more often in the suburban hospitals, where S. pneumoniae and Legionella spp were more common than in the tertiary centre (27.7% v 8.1% and 9.2% v 1.6%, respectively; P = 0.001). Patients in the suburban hospitals were younger and less likely to be in residential care than those in the tertiary centre (mean, 67.6 v 72.6 years and 13.7% v 24.3%, respectively; P ≤ 0.03).

The Australian antibiotic guidelines recommend appropriate investigations to determine aetiology of CAP.3 Our analysis suggests that usefulness of microbiological investigations varies between different patient populations, and their value may be maximised by using algorithms similar to the recommendations for CAP investigations in United States guidelines.4

Contrary to widespread belief that microbiological diagnostic tests for CAP are low-yield and not helpful, we found that aetiology could be established in up to 55% of patients and a change in management made in 15% by using a combination of diagnostic tests. Our findings should be generalisable to other parts of Australia, as the aetiologies we observed mirror those seen in the Australian CAP Study.1