The Natural Progression of Synovial Fluid White Blood-Cell Counts and the Percentage of Polymorphonuclear Cells After Primary Total Knee Arthroplasty

Background: Assessments of the synovial fluid white blood-cell (WBC) count and percentage of polymorphonuclear cells (PMNs) have been reported to be useful in the diagnosis of periprosthetic joint infection. The purpose of this multicenter retrospective study was to evaluate the natural progression of the synovial fluid WBC count, PMN percentage, and total neutrophil count in patients who underwent knee aspiration during the first two years after primary total knee arthroplasty and had no evidence of periprosthetic joint infection.

Methods: From April 1999 to March 2012, 571 patients who presented within the first two years after primary total knee arthroplasty underwent knee aspiration as part of an evaluation for periprosthetic joint infection. Patients were categorized into four groups on the basis of the number of days between surgery and arthrocentesis. The synovial fluid WBC count, PMN percentage, and total neutrophil count were compared among the four time periods with use of separate one-way analyses of variance and Tamhane post-hoc analyses.

Results: Four hundred and fifty-two samples not associated with a periprosthetic joint infection were adequate for analysis. The synovial fluid WBC count, PMN percentage, and total neutrophil count all decreased after the first ninety postoperative days. The synovial fluid WBC count showed an earlier return to a level similar to the two-year time point than the PMN percentage did. The mean total neutrophil count decreased from 2533.2 cells/μL during the first forty-five days to 649.0 cells/μL from forty-six to ninety days, 269.5 cells/μL from three months to one year, and 240.8 cells/μL from one to two years.

Conclusions: The synovial fluid WBC count and PMN percentage changed at different rates over the first two years after total knee arthroplasty, with the WBC count exhibiting an initially more rapid decrease and the PMN percentage demonstrating a more linear decrease. Hence, the total neutrophil count, which combines these two parameters, may provide a better method to identify patients with a periprosthetic joint infection. Values for the synovial fluid WBC count, PMN percentage, and total neutrophil count were all significantly elevated in the early postoperative period, and the use of standard cutoff values for the diagnosis of periprosthetic joint infection can lead to false-positive results.

Level of Evidence: Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.