Utility of Synovial Alpha Defensin Lateral Flow Test in Diagnosing Septic Arthritis in Native Knees.

INTRODUCTION: Septic arthritis of native joints requires prompt diagnosis to prevent joint destruction and sepsis. Traditional diagnostic methods, such as synovial fluid culture and cell counts, are time-consuming and can be unreliable. The Synovasure® Alpha Defensin Lateral Flow Test (Zimmer Biomet, Claymont, DE, USA) detects alpha defensin in synovial fluid and is well-established for diagnosing periprosthetic joint infections. However, its diagnostic value in native joints remains insufficiently studied. METHODS: This prospective cohort study enrolled 25 adults presenting with suspected septic arthritis of the knee. Patients were excluded if they had prior knee arthroplasty or orthopedic implants. Demographics, comorbidities, vitals, and lab values were collected. Patients who tested positive for alpha defensin on aspirated synovial fluid were compared to those who tested negative. Infection was defined as a positive synovial culture, while gout was defined as positive synovial crystals. We calculated sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). RESULTS: Nineteen patients tested positive for alpha defensin, of whom four had culture-confirmed septic arthritis. All six alpha defensin test-negative patients had negative cultures. The test demonstrated a sensitivity of 100% (CI: 39%-100%), specificity of 29% (CI: 11%-52%), PPV of 21% (CI: 16%-25%), and NPV of 100% (CI: 54%-100%). Demographics, comorbidities, and serum inflammatory markers were similar between test-positive and test-negative groups. Alpha defensin test-positive patients had a significantly higher synovial polymorphonuclear neutrophil (PMN) percentage (p<0.001). All nine patients with positive synovial crystals tested positive (p=0.04). DISCUSSION: The high sensitivity and NPV of the Synovasure® Alpha Defensin Lateral Flow Test (Zimmer Biomet) demonstrate its potential utility to rapidly rule out septic arthritis in native knees. However, the test's low specificity warrants caution, as false-positive results may occur with noninfectious inflammatory processes. While the rapid turnaround may aid in early triage and exclusion of infection, confirmatory evaluation with synovial culture remains essential to prevent misdiagnosis and overtreatment.