INTRODUCTION: The diagnosis of periprosthetic joint infection (PJI), particularly during the early postoperative period, remains challenging. The purpose of this multicenter study was to assess the utility of synovial white blood cell (WBC) count and polymorphonuclear leukocyte percentage (PMN%) in the diagnosis of acute PJI at different time intervals after primary total joint arthroplasty (TJA). METHODS: This retrospective study identified 225 patients who underwent a joint aspiration within 90 days of their primary TJA at three institutions. A PJI was defined as revision TJA for infection within one year of aspiration with two positive cultures isolating the same organism. Patients were considered aseptic if they had no reoperation for up to one year after arthrocentesis. Receiver operator characteristic (ROC) curves were used to assess the utility of WBC count and PMN% in the diagnosis of acute PJI at zero to 15, 15 to 45, and 45 to 90 days. Youden's index was utilized to identify the optimal cutoffs at each time interval. A pairwise comparison was performed to compare the area under the curve (AUC) of the two markers. RESULTS: There were 214 patients included. Of these, 81 (37.9%) were infected and 133 (62.1%) were aseptic. The optimal cutoff at zero to 15 days was 21,003 cells/μL for WBC count (AUC 0.937, sensitivity 88.0%, specificity 100%) and 92.0% for PMN% (AUC 0.638, sensitivity 56.0%, specificity 68.4%). Although the diagnostic threshold for WBC count decreased to 5,111 cells/μL at 15 to 45 days and to 2,503 cells/μL at 45 to 90 days, the cutoffs for PMN% did not downtrend in the same fashion (80% at 15 to 45 days
86% at 45 to 90 days). Using pairwise comparison, the AUC for WBC count was higher than that of PMN% at zero to 15 days (P = 0.002), however, there was no difference in AUCs at either 15 to 45 days (P = 0.438) or 45 to 90 days (P = 0.826). CONCLUSION: Based on our findings, it appears that there may be a role for diagnostic thresholds for the identification of acute PJI that are specific to the timing of joint aspiration after primary TJA.