Assessing the diagnostic accuracy of synovial fluid send-out laboratory panels for suspected knee periprosthetic joint infection

Aims: Although synovial fluid send-out laboratory panels are increasingly used in the diagnosis of periprosthetic joint infection (PJI), the impact of transportation delays on the accuracy of synovial fluid white blood cell (WBC) count and polymorphonuclear neutrophil percentage (%PMN) remains unclear. The aim of this study was to assess the accuracy of two send-out synovial panels compared with institutional synovial WBC count and differential in patients evaluated for total knee arthroplasty (TKA) PJI. Methods: Between October 2015 and July 2023, all patients with a suspect TKA PJI with both an institutional synovial cell count and a corresponding next-generation sequencing (NGS) or α-defensin result, derived from separate aliquots of the same aspirate, were assessed. Synovial WBC count, %PMN, synovial CRP, α-defensin, serum CRP, serum ESR, and D-dimer were extracted from institutional and send-out laboratory results. The concordance of synovial WBC count and %PMN values was assessed using the 2018 International Consensus Meeting (ICM) thresholds for chronic PJI. Sensitivity, specificity, positive predictive value, and negative predictive value were calculated to evaluate the diagnostic performance of the institutional and send-out analyses. Results: In total, 212 TKAs (44 (20.8%) next-generation sequencing (NGS), 168 (79.2%) α-defensin) were identified. Relative to institutional analysis, send-out WBC counts were significantly lower (NGS -58.6% (SD 39.2%), α-defensin -4.4% (SD 30.1%); p < 0.001), whereas %PMN was similar for both (NGS -0.1%, α-defensin 1.2%; p = 0.365). Despite these differences, only four (1.9%) send-out WBC counts and nine (4.2%) send-out %PMN values altered ICM minor criteria thresholds, switching from 'Infected' to 'Possibly infected' only once (0.5%). The WBC and %PMN counts from the NGS panel had a sensitivity of 0.923 and a specificity of 1.000, while the α-defensin panel had a sensitivity of 0.970 and a specificity of 0.857 for diagnosing PJI. Conclusion: Synovial fluid cell count and differential values obtained from NGS and α-defensin panels are reliable for diagnosing chronic PJI.