O2089: Low Prevalence of Anaerobic Bacteria in Pediatric Septic Arthritis Makes Obtaining Anaerobic Cultures of Questionable Value

Background: Pediatric acute septic arthritis necessitates urgent identification and treatment to avoid irreversible joint damage if not recognized and treated in a timely fashion. Many centers routinely obtain both aerobic and anaerobic cultures for the purpose of pathogen identification, however the yield of anaerobic culture results in the setting of musculoskeletal infection has been called into question. The goal of this study was to determine the positivity rate of anaerobic cultures collected intraoperatively in pediatric patients with a clinical diagnosis of septic arthritis.

Methods: Patients with a clinical diagnosis of septic arthritis were extracted from a search of musculoskeletal infections at a large tertiary care pediatric hospital from 2007 to 2021. We examined cultures obtained from the operating room or through arthrocentesis. These cultures are routinely obtained in both aerobic and anaerobic culture media at our institution. We assessed the cultures and the clinical scenario surrounding them to assess the utility of anaerobic cultures in the diagnosis and management of pediatric septic arthritis.

Results: We identified 466 cases of pediatric septic arthritis of which only 3 cultures were positive (0.6%) for anaerobic organisms, two involving a hip and one involving a shoulder (Table 1). All cases involved a single isolate. The anaerobic bacteria that were detected included one facultative anaerobe, one strict anaerobe, and one relatively aerotolerant anaerobe (Table 1). Four aerobic organisms grew on the anaerobic cultures. These included Methicillin Resistant S. Aureus (2) and Staphylococcus (2). These four false positives also grew on aerobic culture media. Neither the facultative anaerobe nor the aerotolerant anaerobe grew on aerobic culture media.

Conclusion: Over a 14-year period, only 0.6% (3 cases) of septic arthritis cases yielded positive anaerobic cultures that were not able to be cultured on aerobic media. As such, an anaerobic culture was more likely to culture a bacterium that would have also been also cultured on aerobic media. Additionally, a true positive anaerobic culture that would not have been diagnosed on aerobic cultures only occurs about once every 5 years at our institution. These results suggest that routinely obtaining anaerobic cultures may be of limited value in pediatric septic arthritis. In our sample a positive culture of strict or facultative anaerobe was exceedingly unusual. The false positive rate of anaerobic labs exceeds that of true positive cases. These results provide actionable opportunity to help guide clinician decision making in a more cost-effective and efficient management of pediatric septic arthritis.