Can we identify which patients are likely to have septic arthritis with borderline synovial fluid cell counts?

Objective: Septic arthritis (SA) is a clinical emergency that requires prompt diagnosis and surgical treatment to minimize morbidity and mortality. A synovial fluid cell count (SFCC) greater than 50 000 mm^-3 has been a threshold used to diagnose SA. However, patients with lower cell counts may have culture-positive septic arthritis, resulting in missed or delayed diagnoses. The purpose of this study was to assess the risk of culture-positive SA in patients with synovial fluid analyses that would typically be considered inconclusive and determine what factors may increase the risk of SA. Methods: Patients with SFCC $<$ 50 000 mm^-3 were retrospectively assessed at a single academic institution between 2010 and 2019. Laboratory measures such as white blood cell count (WBC), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), SFCC polymorphonuclear leukocyte percent (PMN), and patient factors and comorbidities were evaluated to determine a threshold for diagnosing SA in this patient population. Results: A total of 199 of 849 patients met inclusion criteria. There were 31 (15.6 %) cases of SA. SFCC and PMN thresholds of 20 000 cell mm^-3 and 70 %, respectively, maximized sensitivity and specificity of SA detection. Prior history of SA, inflammatory arthritis and/or crystalline arthropathy was associated with increased risk of SA. Conclusion: The traditional SFCC threshold of 50 000 cells mm^-3 may not be a reliable diagnostic criterion for all patients. Prior history of SA, inflammatory arthritis, or crystalline arthropathy is associated with increased risks of SA. It may be more appropriate to have lower diagnostic thresholds to clinically diagnose SA in these subsets of patient populations.