Utility of detecting intracellular organisms in identifying the causative organism in pyothorax

Background Intracellular organisms (ICOs), which can be revealed by Gram stain of white blood cells, may result from phagocytosis. To clarify the clinical significance of ICOs, the frequency of ICOs in the pleural fluid was determined and compared with the results of pleural fluid Gram stain and culture. Methods We conducted a retrospective observational study on patients who underwent pleural effusion smear culture tests at Chiba University Hospital between January 1, 2018, and December 31, 2022. We included all patients who underwent pleural fluid smear and culture examinations. Results Of the 1,196 patients included, 137 (11.4%) were culture-positive; of these, 47 patients were Gram-positive (sensitivity 33.6%, specificity 99.9%). ICOs were observed in eight patients, and all were consistent with positive culture results (sensitivity 5.8%, specificity 100%). Of the 62 organisms detected by Gram stain, 54 (86.9%) were confirmed by culture, whereas eight (13.1%) were not confirmed by culture. Conclusions Based on the sensitivity results, detection of ICOs was less valuable than Gram stain for rapid diagnosis. At the organism level, the concordance with culture positivity was 86.9% for Gram stain and 100% for ICO detection. Although ICO detection had limited sensitivity, its high concordance with culture positivity suggests that a positive ICO finding may help identify the causative organism.