Diagnosis of methicillin-resistant Staphylococcus aureus pneumonia: evaluation of presence of intracellular organisms in Gram-stained sputum

Background: Bacteria that aren’t necessarily the primary causative agents of pneumonia may be detected in sputum culture tests. In the diagnosis and treatment of pneumonia, Gram-stained sputum has demonstrated its utility. However, the evaluation for ICO has not been explored. Objective: This study aimed to investigate methicillin-resistant Staphylococcus aureus (MRSA) and evaluate whether the presence of ICO observed in Gram-stained sputum samples enhances the diagnostic certainty of pneumonia and aids in appropriate antimicrobial agent selection. Methods: A retrospective, one: one matched case-control study was conducted from January 1, 2010 to December 31, 2022. Participants: Patients who underwent sputum smear culture testing at Chiba University Hospital were included in the study. Results: A total of 1,075 patients were enrolled in this study, with ICO positivity identified in 65 patients (6.0%). Among these, 30 patients (46.2%) with ICO positivity were diagnosed with pneumonia, while 21 patients (32.3%) with ICO negativity were diagnosed with pneumonia. No statistically significant difference was observed in the occurrence of pneumonia between ICO-positive and ICO-negative patients (p = 0.15, bootstrapped 95 % CI [-0.038, 0.305]). Furthermore, in cases of pneumonia, the presence of ICO did not yield a statistically significant difference in the efficacy of anti-MRSA medications (p = 0.32, bootstrapped 95 % CI [-0.434, 0.292]). Conclusion: The presence of ICO in sputum samples did not demonstrate utility as a diagnostic marker for MRSA pneumonia or in the selection of appropriate antimicrobial agents.