Aims

To identify variables independently associated with same-day discharge (SDD) of patients following revision total knee arthroplasty (rTKA) and to develop machine learning algorithms to predict suitable candidates for outpatient rTKA.

Introduction. Despite several preventive strategies, periprosthetic joint infection (PJI) following total joint arthroplasty (TJA) is still a devastating complication. Early diagnosis and appropriate treatment are crucial to achieve successful infection control, but challenging since there is no test with 100% sensitivity and 100%. Therefore, several national and international guidelines include synovial analysis of joint aspirates as important diagnostic criteria, but cut-off levels for synovial cell count (CC) and polymorphonuclear (granulocyte) percentage (PMN%) are still debatable. The current investigation was performed to analyze the overall accuracy and optimal cut-off of synovial CC and PMN% following total knee (TKA) and total hip arthroplasty (THA). Methods. Between October 2012 and June 2017, all patients with painful TKA or THA, who underwent joint aspiration before revision arthroplasty were included in this retrospective study. From aspirated synovial fluid, leukocyte esterase activity, leukocyte CC and PMN% were determined, and specimens were sent for bacterial culture. A total of 524 preoperative joint aspirations (255 hips, 269 knees) were enrolled for final analysis. For 337 patients, the synovial CC and PMN% could be measured by the laboratory. From those patients, 203 patients were scheduled for aseptic revision, and 134 patients for septic revision arthroplasty according to the MSIS criteria for PJI. Specificity (SP), sensitivity (SE), positive predictive value (PPV), negative predictive and overall accuracy were measured for CC and PMN%. The optimum cut-off value was calculated by the ROC and the value giving the AUC, achieving the best possible level of sensitivity and specificity. Results. The best cut-off level for PJI of all study patients was 2582 leukocytes/μL (Se 80.6%, Sp 85.2%) and a PMN% of 66.1% (Se 80.6%, Sp 83.3%). The chosen cut-off levels for PJI of TKA was 1630 leukocytes/μL (Se 83.6%, Sp 82.2%) and a PMN% of 60.5% (Se 80.3%, Sp 77.1%). The optimal cut-off values for PJI of THA was 3063 leukocytes/μL (Se 78.1%, Sp 80.0%) and a PMN% of 66.1% (Se 82.2%, Sp 82.4%). Conclusions. Synovial cell count and polymorphonuclear percentage are sensitive methods for diagnosing PJI with differences in cut-off levels for THA and TKA. We suggest considering the cut-off levels of CC and PMN% from aspirates of TKA at 1600/μL and 60%, respectively, as possible PJI. For THA, the cut-off levels of CC and PMN% are at 3000/μL and 66%, respectively