Delta Neutrophil Index in Suspected Septic Arthritis: A Diagnostic Accuracy Study

Background: Septic arthritis is an orthopedic emergency requiring prompt diagnosis and treatment to prevent joint destruction and systemic complications. Standard inflammatory markers such as CRP, ESR, and WBC lack specificity, often leading to diagnostic uncertainty, especially in differentiating septic arthritis from inflammatory arthropathies. The Delta Neutrophil Index (DNI), reflecting immature granulocyte levels, may offer improved diagnostic performance. Questions/purposes: We aimed to evaluate (1) Does serum DNI provide superior diagnostic accuracy compared with CRP, ESR, and WBC for culture-positive septic arthritis? (2) What sensitivity and specificity are achieved at an optimal DNI threshold? Methods: In this singlecentre retrospective diagnostic cohort study, 71 patients who underwent surgery for suspected septic arthritis between November 2022 and March 2025 were enrolled. Preoperative serum biomarkers were obtained; synovial aspirate appearance and intraoperative cultures served as the reference standard. PCT measured in 33/71 patients. Diagnostic performance was assessed with ROC analysis; AUCs were compared with the bootstrapadapted DeLong test, and multivariable logistic regression adjusted for age, sex, and affected joint. Results: DNI values were significantly higher in culture-positive patients (p = 0.0002). While CRP and ESR showed moderate diagnostic value, DNI exhibited the strongest correlation with positive cultures and purulent synovial fluid. The ROC AUC for DNI was significantly higher than those of CRP, ESR and WBC (p < 0.001), suggesting better diagnostic performance. A DNI cut-off value of 0.6 yielded the highest sensitivity and specificity among all markers. Importantly, DNI values did not increase significantly in patients with inflammatory arthritis flares, unlike CRP. Conclusions: The Delta Neutrophil Index is a rapid, cost-effective, and more specific biomarker than conventional inflammatory parameters for diagnosing septic arthritis. Its use may help differentiate septic arthritis from inflammatory arthropathies and reduce unnecessary surgical interventions. Level of Evidence: Level III, retrospective diagnostic study.