Discordance of the Urinary and Pleural Fluid Antigen Test and False Positive for Streptococcus pneumoniae in Empyema Secondary to Necrotizing Bacterial Pneumonia

Empyema is a severe complication of pneumonia with high morbidity and mortality rates. Rapid diagnosis and tailoring of antibiotic therapy are crucial to treatment success for these severe bacterial lung infections. A Streptococcus pneumoniae (S. pneumonia) antigen test drawn from the pleural fluid rather than a urine sample has been found to have equivalent diagnostic utility to the urinary antigen test. Discordance between these tests is rare. We report a case of a 69-year-old female with CT imaging findings consistent with empyema and a bronchopulmonary fistula. A rapid S. pneumonia antigen test was negative from the urinary sample but positive when drawn from a patient's pleural fluid sample. Final pleural fluid cultures resulted in Streptococcus constellatus (S. constellatus). This case demonstrates discordance between the results of urinary and pleural fluid S. pneumoniae antigen tests, representing a potential pitfall in using rapid antigen testing on pleural fluid samples. False positives for the S. pneumoniae antigen in patients with viridans streptococci infections have been documented due to the cross-reactivity of cell wall proteins in different streptococcal species. Physicians encountering bacterial pneumonia of unknown etiology complicated by empyema should understand the potential for discordance and false positives using this diagnostic method.