TAILORED DIAGNOSTICS IN JOINT ARTHRITIS: INSIGHTS FROM SYNOVIAL FLUID LEUKOCYTE COUNT, POLYMORPHONUCLEAR CELLS, PCR, AND CULTURE

Aim Native (JI) and prosthetic joint infections (PJI) pose major challenges in orthopaedics. Timely identification of the causative pathogen is critical to improve patient outcome. Syndromic multiplex PCR on synovial fluid offers rapid and accurate pathogen detection. A multicenter study is currently being conducted evaluating diagnostic and clinical impact of PCR1 in patients with symptoms of joint arthritis. The predictive value of synovial leukocyte count (WBC) and % polymorphonuclear cells (PMN) for infection diagnosis and microbial positivity was also investigated. Methods PCR1 next to culture, leukocyte count and % PMN was performed on 114 synovial fluid samples if the threshold exceeded 50.000 WBC/µL and 90% PMN for JI (Sanjo criteria, 2023) or 3.000 WBC/µL and 80% PMN for PJI (European Bone and Joint Infection Society criteria, 2021). In 50 synovial fluid samples only leukocyte count, % PMN and culture were analyzed. Logistic regression models were constructed to estimate the probability for infection based on synovial fluid WBC count, % PMN, culture and PCR1. The predictive value for positive microbial diagnosis based on synovial fluid WBC count and % PMN was also assessed. Results Mean WBC count and % PMN were higher in patients with joint infection, but WBC thresholds were also met in patients without joint arthritis. Only for patients with prosthetic joints, PMN better distinguished between infection (>80%) and non-infectious arthritis (<80%). PCR outperformed culture for correct diagnosis of acute infection both in patient with native (PCR: 81% vs. culture: 72%) as well as prosthetic joints (PCR: 88% vs. culture: 80%). The use of PCR next to culture further increased diagnostic accuracy. Both culture and PCR showed low diagnostic accuracy in patients suspected for chronic PJI (53%). Logistic regression revealed PCR as the strongest predictor for JI (p-value < 0.05), with WBC count partially predictive (p-value < 0.05) and PMN non-contributory (p-value = 0.32). In chronic PJI, culture (p-value= 0.04) outperformed PCR (p-value = 0.37) and WBC count or % PMN were minimally useful (p-value > 0.05). Weak to no correlation was observed for leukocyte count and % PMN as predictors of PCR or culture positivity. Conclusions Our current data underscore the high diagnostic accuracy of PCR1 in patients with acute arthritis. WBC count and % PMN add limited value to infection diagnosis nor predict PCR or culture positivity, especially in chronic PJI. Acknowledgements Supported by bioMérieux, France. 1 BIOFIRE® Joint Infection Panel (bioMérieux, France)