Pathogen or contaminant? Distinguishing true infection from synovial fluid culture contamination in patients with suspected septic arthritis

Background Isolation of bacteria from synovial fluid (SF) is the gold standard for diagnosis of septic arthritis (SA). Contamination results in misdiagnosis and mismanagement. This study identifies clinical characteristics, microbiology, and outcomes of patients with contaminated SF and compares them with patients with true SA. Methods We conducted a retrospective study including all patients aged 18 and older admitted to a single, tertiary-care hospital between 1998 and 2015 with suspected SA and positive SF cultures. Contamination cases were determined by infectious disease specialists involved in the patients’ care and a clinical course inconsistent with SA. Results 398 patients with true SA and 22 with contaminated SF were identified. The SA group was younger (60.9 vs. 75.6 years; p < 0.01), had higher peripheral polymorphonuclear lymphocytes (78.0 vs. 69.4%; p < 0.01) and SF white blood cell count (91.7 vs. 25.6K/mL; p = 0.02), and longer mean length of stay (10.9 vs. 6.7 days; p = 0.02). The average time to positive culture was longer in the contaminated group (3.62 vs. 1.4 days; p < 0.01). The SA group was less likely to receive a new rheumatologic diagnosis within 1 year (3.0 vs. 36.4%; p < 0.01). Conclusion This is the first study of its kind looking at clinical features and outcomes of patients with contaminated SF. These patients present with less severe disease, have better outcomes, and receive new rheumatologic diagnoses in more than a third of cases within 1 year. We recommend a conservative approach for patients with suspected contaminated SF, mild symptoms, and no bacterial growth within the first 48 h.