Clinical predictive factors for septic arthritis of the knee in children: A cohort study

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Objective Criteria to differentiate pediatric knee joint effusion remained inexpedient. This study aimed to establish a prediction rule to distinguish infection from other inflammatory arthritis in children presenting with knee pain. Methods An ambi-directional cohort was conducted at the university hospital from 1999 to 2021 by including children aged ≤ 15 years with knee pain/swelling, complete physical examination, knee imaging, laboratory, and/or synovial fluid analysis. Contaminated synovial fluid culture and incomplete medical records were excluded. Diagnosis of septic knee (positive joint fluid culture or synovial white blood cell counts > 50,000 cells/mm 3 ) or other inflammatory joints were retrieved. Baseline characteristics, physical examination, and laboratory investigations were analyzed. The probability of septic arthritis was calculated based on numbers of predictors from the final model. Results From 48 patients (average age 7.0 ± 4.0 years, and 24 (50%) unilateral involvement), the incidence of septic knee was 9 patients (18.8%), and 39 patients with other inflammatory arthritis. Multivariate logistic regression identified three predictors: unable to bear weight, ballottement, and erythrocyte sedimentation rate (ESR) ≥ 75 mm/h (adjusted odds ratio (OR) 36.2, 19.4, and 31.9, respectively, the model chi-square p < 0.0001, with area under receiver operating characteristic curve = 0.9188). Estimated probability of having septic knee according to 1, 2, and 3 predictors was 27.3% (95% confidence interval (CI) 13.3–45.5%), 61.5% (95% CI 31.6–86.1%), and 100% (95% CI 15.8–100%), respectively. The optimal cutoff value from sum of estimated ORs was ≥ 51.29 (at least 2 predictors) with sensitivity 88.89%, specificity 87.18%. Conclusions The clinical predictive factors of septic knee in children may be practically determined by unable to bear weight, ballottement, and ESR at least 75 mm/h. The probability of septic knee increases, especially for positive 2–3 factors. Further large studies would benefit for external validating this prediction rule.

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