Markers of culture positivity and a novel scoring system in native knee septic arthritis.

backgroundAlthough synovial fluid culture is the diagnostic gold standard for septic arthritis, culture negativity is common and reported rates range widely, from approximately 20% to 60%, depending on diagnostic criteria, prior antibiotic exposure, and culture methodology. Existing prediction scores focus on differentiating septic from aseptic arthritis but do not predict culture positivity in patients with established septic arthritis. We developed and internally evaluated the Istanbul Septic Arthritis Score (ISAS) to predict synovial fluid culture positivity in native adult knee septic arthritis.

methodsThis retrospective cohort study included 198 adult patients (≥ 18 years) treated for native knee septic arthritis at a tertiary referral center between January 2006 and December 2025. Septic arthritis was diagnosed using the Newman classification criteria (Grade A: culture-positive; Grade C: culture-negative with intraoperative purulence and clinical findings). Demographic, clinical, serum laboratory, and synovial fluid parameters were analyzed. Multivariable logistic regression identified independent predictors of culture positivity, which were used to develop a simplified clinical scoring system.

resultsOf 198 patients, 60 (30.3%) had culture-positive and 138 (69.7%) had culture-negative septic arthritis. Three independent predictors of culture positivity were identified: serum C-reactive protein (OR = 1.006 per mg/L, p < 0.001), synovial fluid leukocyte count (OR = 1.041 per 10,000/mm³, p = 0.010), and younger age (OR = 0.980 per year, p = 0.021). The ISAS score (range 0-7 points) demonstrated moderate discriminative ability (AUC = 0.713, 95% CI: 0.631-0.795, p < 0.001), equivalent to the full multivariable model. Using a cut-point of ≥ 5, two risk groups showed clear separation in culture positivity: a lower-risk group (0-4 points, 19.5%) and a higher-risk group (5-7 points, 52.3%) (p < 0.001).

conclusionISAS is a simple, preliminary tool that may help estimate synovial fluid culture positivity in native adult knee septic arthritis and is intended to support, not replace, clinical judgement. In higher-risk patients (ISAS ≥ 5), broader empiric antimicrobial coverage, extended culture incubation, and molecular diagnostic methods may warrant consideration alongside clinical judgement and local microbiological protocols. As this score was derived from a single-center retrospective cohort, external validation in multicenter prospective settings is essential before routine clinical use. CLINICAL

trial registrationNot applicable.