Synovial Periprosthetic Infection Markers Show High Variability in Different Clinical and Microbiological Settings

<b>Background/Objectives:</b> Accurately diagnosing a periprosthetic joint infection (PJI) represents a challenging and complex task. Especially in the case of low-grade infections, important diagnostic modalities may be inconclusive and synovial markers such as white blood cell count (WBC) and polymorphonuclear percentage (PMN) gain relevance. We therefore aim to assess WBC and PMN in different clinical and microbiological settings. <b>Methods:</b> We performed a retrospective analysis of 115 patients with a diagnosed PJI. WBC and PMN were compared between patients with low- and high-virulent infections, negative and positive histology, symptom duration ≤ 4 weeks and >4 weeks, and positive and negative pathogen detection. <b>Results:</b> Synovial WBC was significantly higher in patients with successful pathogen detection (42.44 [87.0] G/L vs. 16.35 [32.0] G/L; <i>p</i> < 0.01), as was PMN (86.0 [60.0]% vs. 91.0 [89.0]%; <i>p</i> < 0.01). PJIs with high-virulent pathogens showed higher WBC compared to low-virulent pathogens (58.27 [102.0] G/L vs. 27.27 [46.0] G/L; <i>p</i> < 0.01). Patients with onset of symptoms ≤ 4 weeks demonstrated higher WBC (58.27 [112.0] G/L vs. 16.42 [46.0] G/L]; <i>p</i> < 0.01) as well as higher PMN (91.5 [9.0]% vs. 88.0 [20.0]%); <i>p</i> = 0.042). Cases with negative histology showed significantly lower WBC (16.73 [44.0] G/L vs. 42.86 [87.0] G/L; <i>p</i> < 0.01) and lower PMN (86.0 [67.0]% vs. 91.0 [9.0]%; <i>p</i> = 0.036). <b>Conclusions:</b> WBC and PMN show high variability and appear to be influenced by virulence, histology, onset of symptoms, and pathogen detection.