Purpose: Detection of infection persistence during the two-stage exchange of the knee for periprosthetic joint infection is challenging. Synovial fluid culture (SFC) and synovial white blood cell count (SWBCC) before joint reimplantation are widespread diagnostic means for this indication. The sensitivity and specificity of SFC and of SWBCC for infection persistence before planned reimplantation were evaluated.
Methods: 94 two-stage exchanges of the knee with synovial fluid aspiration performed after a drug holiday of at least 14 days and before reimplantation or spacer exchange (planned reimplantation) were retrospectively analyzed. Only cases with at least 3 intraoperative samples at planned reimplantation were included. SFC and SWBCC were compared to pathogen detection (SFC(culture)/SWBCC(culture)) and to histopathological signs of infection persistence (SFC(histo)/SWBCC(histo)) from intraoperative samples at planned reimplantation. For SFC, the sensitivity and specificity were calculated. For SWBCC, the optimal cut-off value with its sensitivity and specificity was calculated with the Youden-Index.
Results: Sensitivity and specificity of SFC(culture) were 0.0% and 98.9%. Sensitivity and specificity of SFC(histo) were 3.4% and 100%. The optimal cut-off value for SWBCC(culture) was 4450 cells/μl with a sensitivity of 50.0% and a specificity of 86.5%. The optimal cut-off value for SWBCC(histo) was 3250 cells/μl with a sensitivity of 35.7% and a specificity of 92.9%.
Conclusion: The detection of infection persistence remains challenging and a consented approach is lacking. The results do not warrant the routine performance of SFC during the two-stage exchange at the knee. SWBCC can be used to confirm infection persistence at high cut-offs, but they only occur in few patients and are therefore inappropriate for the routine use.