Utility of Gram Stain to Identify the Pathogen in Methicillin-resistant Staphylococcus aureus Pneumonia

Background: Early de-escalation of empiric therapy for pneumonia could be considered if gram stain results are reliable to rule out a causative pathogen. Concordance of gram stains and final culture results are variable, but may be higher with gram-positive organisms. The purpose of this study is to determine the utility of gram stain in predicting growth of methicillin-resistant Staphylococcus aureus (MRSA) in hospitalized patients with MRSA pneumonia.

Methods: This was a retrospective, observational study of patients admitted to Norton Healthcare, a 1537 licensed adult bed community health-system between January-June 2012. Patients were identified by positive MRSA respiratory cultures, and included if they demonstrated signs and symptoms of pneumonia. Gram stain description, specimen collection method, and time from obtaining cultures to initial reporting of S. aureus and final susceptibilities were examined.

Results: The presence of gram-positive cocci was described in 77 of 84 patients with MRSA pneumonia yielding a sensitivity of 91.7%. Expanding the descriptive criteria to all gram-positive organisms increased the sensitivity to 95.2%. Gram stain sensitivity was 87.5%, 96.7%, and 89.5% for tracheal aspirates, sputum, and bronchoalveolar lavage cultures, respectively. Sensitivity of gram stain was not different between upper and lower respiratory specimen collection methods (92.3% and 89.5% respectively, p = 0.65). Microbiology reports identifying S. aureus as the pathogen were reported within 2 days for 93.8% of the cultures.

Conclusion: Gram stains may be useful in guiding de-escalation of anti-MRSA therapy in patients with pneumonia. Importantly, most cultures reported growth of S. aureus within 2 days, thus minimizing the risk of harm if patients were inappropriately de-escalated. Institution specific factors such as prevalence of MRSA, experience of microbiology technicians, and local surveillance culture practices must be taken into consideration.