Introduction: Although there is no absolute diagnostic test for PPI, synovial leukocyte counts and neutrophil percentages have been reported to have high sensitivity and specificity making them valuable tests for diagnosis. However, no cut off value for the latter is agreed upon. This study intends to evaluate to define definite cut-off values for preoperative synovial fluid leukocyte count and PMN percentage that allow physicians to diagnose infection in a prosthetic joint.

Methods: We analyzed synovial fluid aspirated preoperatively from 593 total knee arthroplasties (161 infected; 432 aseptic) from three different institutions. Using ROC curves, we determined cut-off values with optimal accuracy in diagnosis of infection for fluid leukocyte count and PMN percentage. The sensitivity, specificity, and predictive values were calculated for the above cutoff values.

Results: The synovial fluid leukocyte count was higher in patients with PPI (median, 31 × 103 cell/μl) compared to aseptic joint arthroplasties (median, 0.219 × 103 cells/μl)(p< 0.0001). Similarly, the neutrophil percentage was higher in patients with PPI (median, 90%) than in those with noninfected joints (median, 26%)(p< 0.0001). The cut-off values for optimal accuracy in diagnosis of infection were 1760 cells/μl for fluid leukocyte count and 73% for PMN percentage. A fluid cell count > 1760 cells/μl had a PPV of 99% and NPV of 88%, while a neutrophil percentage > 73% yielded a PPV of 96% and NPV of 91%. Chi-square analysis revealed the fluid WBC count and the neutrophil percentage to be significantly associated with the diagnosis of infection (p< 0.0001).

Discussion: Synovial fluid leukocyte count and differential are useful adjuncts to ESR and CRP in the preoperative diagnosis for infection in TKA. This study has identified a cut-off value for leukocyte count and neutrophil percentage that can be used to diagnose infection in a prosthetic joint.