Pneumonia is a leading cause of mortality secondary to infectious agents in the United States. There have been different attempts to categorize pneumonias, and the American Thoracic Society (ATS) has published its classifications: outpatient with no comorbidities, outpatient with cardiopulmonary disease or other modifying factors, inpatients not admitted to the intensive care unit, and patients admitted to the intensive care unit. The ATS stated that patients with severe community-acquired pneumonia require a specific therapeutic approach. In addition, ATS and other organizations recommend that patients with community-acquired pneumonia should have sputum analysis by Gram stain and culture, cultures of normally sterile fluids such as blood and pleural effusions, and serologic testing. These recommendations have been questioned because some study results have suggested that testing had no impact on patients' outcomes. Rello and associates studied the impact of microbiologic testing on therapeutic decisions and outcomes in patients with severe community-acquired pneumonia.

The study was a retrospective analysis of data collected in a prospective manner. All patients admitted to intensive care units in two teaching hospitals in Spain who met the criteria for the diagnosis of community-acquired pneumonia were enrolled in the study. All patients with this diagnosis were empirically treated with antibiotics on admission to the hospital. Blood cultures, serologic studies for influenza A and B, parainfluenza, and respiratory syncytial viruses were obtained. The serologic testing was repeated at follow-up for most patients. Patients also were tested for other common etiologic agents, including Legionella. Sputum samples were obtained from patients intubated for mechanical ventilation. These test results were used to determine if any change in the initiating antibiotic therapy was necessary.

Initially, 210 patients were enrolled in the study. Of these, 106 required intubation, and 81 patients needed noninvasive mechanical ventilation. Median age was 60 years. A microbiologic diagnosis of pneumonia was established in 117 patients. The most common pathogens discovered were Streptococcus pneumoniae, Legionella pneumophila, and Haemophilus influenzae. Pseudomonas and Legionella were more common in patients who were intubated and also were more lethal than the other microbial agents. Based on test results, 41.6 percent of patients required changes to the antibiotic regimen. Five percent of these changes occurred because the initial antibiotic therapy did not cover the microbial isolates. The most frequently cited reason for changing antibiotics was an effort to simplify the regimen.

The authors conclude that microbiologic testing is justified in patients with severe community-acquired pneumonia. These tests can assist in directing appropriate antibiotic therapy and affect patient outcomes. These findings also suggest that, because Pseudomonas and Legionella are more common in patients who are intubated, empiric therapy should be used to treat these two microbials.