Background. Diagnostics of infectious complications in joint replacement surgery remains a significant challenge, particularly when microbiological analysis of biological material fails to reveal pathogen growth. The aim of the study was to determine threshold values for C-reactive protein, interleukin-6, and presepsin levels, and to assess their diagnostic value in detecting periprosthetic joint infection. Methods. A prospective cohort single-center blinded study was conducted involving cases of revision arthroplasty for periprosthetic joint infection (PJI) and aseptic prosthetic loosening. The study included 66 patients divided into two groups: Group I (n = 17), with confirmed PJI using the 2018 ICM criteria, and Group II (n = 49), with aseptic prosthetic loosening. Synovial fluid samples were subjected to bacteriological and cytological analysis, measuring levels of C-reactive protein (CRP), presepsin, and interleukin-6 (IL-6). ROC analysis, sensitivity, specificity, accuracy, and threshold values were determined for laboratory data. Results. The highest diagnostic accuracy in distinguishing between PJI and aseptic loosening was observed in the leukocyte count in synovial fluid (AUC 0.928; 95% CI: 0.837-0.977, p0.0001). Elevated synovial CRP levels were associated with infection, with an AUC of 0.776 (95% CI: 0.656-0.870, p = 0.0004), and IL-6 had an AUC of 0.712 (95% CI: 0.583-0.820; p = 0.0048). Presepsin levels, however, showed no significant difference between groups (AUC 0.582; 95% CI: 0.453-0.703; p = 0.3344). Threshold values were set at 5.6 mg/l for CRP, 1212.0 pg/ml for presepsin, and 988.5 pg/ml for IL-6. Sensitivity, specificity, and accuracy for PJI diagnosis were determined for CRP at 62.5%, 85.7%, and 80.0%; for IL-6 at 87.5%, 63.0%, and 69.4%; and for presepsin at 43.8%, 79.6%, and 70.8%, respectively. Conclusion. In cases where synovial leukocyte counts are at borderline levels, the additional assessment of synovial fluid cellular composition and simple, cost-effective markers such as synovial CRP and IL-6 may be recommended to confirm PJI.
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