The Mark Coventry Award: Diagnosis of Early Postoperative TKA Infection Using Synovial Fluid Analysis

Abstract

Background Synovial fluid white blood cell count is useful for diagnosing periprosthetic infections but the utility of this test in the early postoperative period remains unknown as hemarthrosis and postoperative inflammation may render standard cutoff values inaccurate. Questions/purposes We evaluated the diagnostic performance of four common laboratory tests, the synovial white blood cell count, differential, C-reactive protein, and erythrocyte sedimentation rate to detect infection in the first 6 weeks after primary TKA. Methods We reviewed 11,964 primary TKAs and identified 146 that had a knee aspiration within 6 weeks of surgery. Infection was diagnosed in 19 of the 146 knees by positive cultures or gross purulence. We compared demographic information, time from surgery, and the laboratory test values between infected and noninfected knees to determine if any could identify infection early postoperatively. Receiver operating characteristic curves were constructed to determine optimal cutoff values for each of the test parameters. Results Synovial white blood cell count (92,600 versus 4200 cells/lL), percentage of polymorphonuclear cells (89.6% versus 76.9%), and C-reactive protein (171 versus 88 mg/L) were higher in the infected group. The optimal synovial white blood cell cutoff was 27,800 cells/lL (sensitivity, 84%; specificity, 99%; positive predictive value, 94%; negative predictive value, 98%) for diagnosing infection. The optimal cutoff for the differential was 89% polymorphonuclear cells and for C-reactive protein 95 mg/L. Conclusions With a cutoff of 27,800 cells/lL, synovial white blood cell count predicted infection within 6 weeks after primary TKA with a positive predicted value of 94% and a negative predictive value of 98%. The use of standard cutoff values for this parameter (* 3000 cells/lL) would have led to unnecessary reoperations.

Is Synovial C-reactive Protein a Useful Marker for Periprosthetic Joint Infection?

“…We found no advantage to the use of synovial-fluid CRP over serum CRP in the diagnosis of PJI; moreover, in 15% of patients, the synovial-fluid test could not be performed, perhaps making it of even lower utility. Thus, collected synovial fluid is best used to obtain a cell count with a differential and cultures [3,10,16,21,22,27,28,31,32]; diverting the sample for the measurement of CRP does not appear warranted at this time.…”

“…Acute infection was defined as presentation within 6 weeks of an index procedure [3,10,13,28,30,34] and any infection beyond that time was classified as chronic. The patients in the infected and noninfected groups had similar demographics [7] ( Table 1).…”

Swab Cultures Are Not As Effective As Tissue Cultures for Diagnosis of Periprosthetic Joint Infection

“…Tissue and swab culture results were not considered as part of gold standard diagnosis of PJI, as we wanted to remove any variables whose accuracy we were testing as part of the current study. Thus, this modified version of the MSIS criteria was utilized for clinical definition of PJI: (1) there was a sinus tract communicating with the prosthesis; or (2) a pathogen was isolated by culture from two separate fluid samples obtained from the affected prosthetic joint; or (3) when at least three of the following five criteria existed (the MSIS criteria require four of six, but we excluded the criteria of five or more neutrophils on frozen section, thus leaving us with three of five criteria as diagnostic): elevated serum erythrocyte sedimentation rate (ESR) and serum CRP concentration, elevated synovial white blood cell (WBC) count, elevated synovial polymorphonuclear neutrophil percentage (PMN%), presence of purulence in the affected joint, or isolation of a microorganism in one culture of periprosthetic fluid (preoperative [3,22]. Acute postoperative PJI was diagnosed if infection occurred within 6 weeks of index primary joint arthroplasty [3].…”

New Definition for Periprosthetic Joint Infection: From the Workgroup of the Musculoskeletal Infection Society

“…In patients with acute infections, the levels of synovial cell count and PMN% are much higher (approximately 20,000 cells/lL and 89%, respectively). Acute infections are defined as less than 3 months from index surgery or from the onset of symptoms [1]. The levels of synovial cell count and PMN% in the infected hip arthroplasty are not well delineated.…”