Synovial Absolute Neutrophil Count as a Superior Diagnostic Biomarker for Acute Periprosthetic Joint Infection in Hip and Knee Arthroplasty with Low-Virulence Organisms.

Abstract

Diagnosing acute periprosthetic joint infection (PJI) soon after surgery presents a challenge because the inflammation caused by the procedure can mimic that of an infection. Key diagnostic measures include synovial white blood cell (WBC) count, polymorphonuclear percentage (PMN%), and alpha-defensin (AD). Nonetheless, the precise thresholds, particularly for infections with low virulence, remain ambiguous. This research assessed the effectiveness of synovial absolute neutrophil count (ANC) in diagnosing acute PJI in hip and knee joints and established specific thresholds based on the procedure and the organism involved. An examination was carried out on 502 patients who underwent joint aspiration within a 90-day period following their primary total joint arthroplasty, conducted from March 2011 to August 2022 at a tertiary arthroplasty center. Of 502 patients, 123 (24.5%) had an acute PJI. The diagnosis of PJI was established according to the criteria set by the 2018 International Consensus Meeting. Receiver operating characteristic (ROC) curves were used to assess synovial fluid WBC, PMN%, ANC, and AD, aiming to determine the optimal cutoff values for total knee arthroplasty (TKA) and total hip arthroplasty (THA), with differentiation according to the virulence of the organism. For TKA, optimal thresholds were WBC 5,162 cells/μL, PMN% 88.5%, AD 1.7, and ANC 2,881 cells/μL (AUC 0.98; sensitivity 96.8%, specificity 95.5%). For THA, thresholds were WBC 6,273 cells/μL, PMN% 87.2%, AD 1.4, and ANC 3,860 cells/μL (AUC 0.98; sensitivity 95.4%, specificity 94.1%). Organism-specific analysis showed lower synovial values in culture-negative and coagulase-negative Staphylococcus infections and higher values in Staphylococcus aureus and antibiotic-resistant strains. The ANC demonstrated improved diagnostic sensitivity for detecting low-virulence infections compared with other markers. Synovial ANC is a cost-effective, accessible, and highly accurate diagnostic tool for acute PJI in both hip and knee arthroplasty. It showed improved diagnostic performance in low-virulence infections, and the procedure-specific thresholds identified in this study support its integration into early diagnostic protocols to improve detection, enable timely intervention, and reduce reliance on costly assays such as AD.