Sensitivity of synovial fluid analysis in hematogenous native joint septic arthritis – overall accuracy and pathogen-specific differences

Abstract This study aimed to evaluate the sensitivity of synovial fluid (SF) white blood cell count (SF-WBC), SF polymorphonuclear neutrophil percentage (PMN%), and the SF/serum glucose ratio in a study population with a definitive diagnosis of hematogenous native joint septic arthritis (SA) and explore factors influencing their performance, including the causative pathogen. We conducted a retrospective register and medical record study of 57 patients with culture-verified hematogenous native joint SA identified through ICD-codes between 2016 and 2019 in western Sweden. All patients included had documented SF-WBC analysis. Clinical data and SF analyses were compared between patients above and below cutoffs for SF-WBC (50,000 cells/mm 3 ) and SF/serum glucose ratio of 0.5. SF findings were also compared between S. aureus and other pathogens. The sensitivity of SF-WBC ≥ 50,000 cells/mm 3 was 51%, SF/serum glucose ratio < 0.5 was 57% and PMN% ≥ 70 was 95%. Patients with SF-WBC ≥ 50,000 cells/mm 3 were significantly older. SA caused by S. aureus were associated with lower PMN% and higher SF/serum glucose ratio compared to other organisms. While PMN ≥ 70% had a high sensitivity for native joint SA, the conventional cutoffs for SF-WBC and SF/serum glucose ratio showed poor sensitivity. SA caused by S. aureus may be associated with a less pronounced inflammatory response compared with other pathogens, a finding that warrants further investigation.