SEROLOGY ALONE IS INSUFFICIENT: PREOPERATIVE ASPIRATION NEARLY ELIMINATES DIAGNOSTIC UNCERTAINTY IN PERIPROSTHETIC JOINT INFECTION

Background Accurate diagnosis of periprosthetic joint infection (PJI) remains a major challenge in arthroplasty. While guidelines emphasize synovial fluid aspiration, its incremental value beyond serum inflammatory markers in routine clinical practice remains poorly defined. This study evaluated the diagnostic performance of aspiration components and quantified their added value beyond serology using Musculoskeletal Infection Society (MSIS) criteria as the reference standard. Methods A retrospective cohort study was conducted of 1,160 patients evaluated for PJI. 43 patients with an open sinus tract and 19 patients with inconclusive MSIS criteria were excluded, leaving a final analytic cohort of 1,098 patients. Patients were stratified by whether preoperative aspiration was performed (aspirated: n = 661; not aspirated: n = 437). Among aspirated cases, the diagnostic performance of synovial white blood cell count (WBC >3,000 cells/µL), polymorphonuclear percentage (PMN% >80%), and synovial fluid culture was evaluated using sensitivity, specificity, and area under the receiver operating characteristic curve (AUC). Multivariable discrimination was compared between models using serology alone (ESR, CRP) versus combined serologic and aspiration parameters. Results Patients undergoing aspiration demonstrated higher inflammatory markers (ESR: 36.2 vs 19.2 mm/hr; CRP: 5.32 vs 1.19 mg/dL; both p<0.001) and were more likely to meet MSIS criteria for PJI (39.6% vs 2.5%, p<0.001). Among aspirated patients, synovial WBC >3,000 cells/µL demonstrated excellent diagnostic performance (sensitivity 90.5%, specificity 94.7%, AUC 0.926), while PMN% >80% showed strong discrimination (sensitivity 75.8%, specificity 84.6%, AUC 0.802). Synovial fluid culture demonstrated high specificity but limited sensitivity (sensitivity 51.5%, specificity 97.7%, AUC 0.725). A composite aspiration-positive model achieved the highest standalone aspiration performance (sensitivity 92.7%, specificity 84.5%, AUC 0.886). A multivariable serology-only model incorporating ESR and CRP demonstrated strong diagnostic discrimination (AUC 0.939). Among aspirated patients with complete synovial data, incorporation of aspiration-derived variables increased diagnostic performance to near-perfect discrimination (AUC 0.993). Conclusion Synovial biomarkers, rather than culture alone, drive the diagnostic utility of preoperative aspiration in PJI. While serum inflammatory markers demonstrate strong baseline performance, incorporation of synovial WBC and PMN% substantially improves diagnostic discrimination and nearly eliminates residual uncertainty. Preoperative aspiration should be considered essential rather than adjunctive in the diagnostic workup of suspected PJI, reinforcing a multimodal strategy for infection diagnosis.