How Well Do the New EULAR/ACR Classification Criteria Capture Pediatric Chronic Nonbacterial Osteomyelitis? A Multicenter Pediatric Cohort Study

BACKGROUND: Chronic nonbacterial osteomyelitis (CNO) is a rare autoinflammatory bone disorder with heterogeneous presentations, and no disease-specific biomarker exists, making diagnosis challenging. Recently, the European Alliance of Associations for Rheumatology (EULAR) and the American College of Rheumatology (ACR) proposed new classification criteria; however, their performance relative to the established Bristol criteria has not been systematically validated. METHODS: We conducted a retrospective, multicenter cohort study across 10 pediatric rheumatology centers, including 114 children with CNO and 116 controls with mimicking conditions. Demographic, clinical, laboratory, imaging, and biopsy data were systematically collected. Each participant was assessed according to both the EULAR/ACR classification and the Bristol diagnostic criteria. Sensitivity, specificity, predictive values, likelihood ratios, receiver operating characteristic (ROC) analysis, and agreement statistics were calculated using expert clinical diagnosis as the reference standard. RESULTS: The EULAR/ACR criteria demonstrated 97.4% sensitivity and 81.0% specificity, compared with 79.8% and 80.2% for the Bristol criteria. The negative likelihood ratio was markedly lower for the EULAR/ACR criteria (0.03 vs. 0.25), underscoring their strong rule-out capacity. Concordance between systems was substantial (κ=0.66). Discordant cases showed that the EULAR/ACR criteria better identified atypical phenotypes such as unifocal or nonclavicular disease. Elevated ESR supported the diagnosis of CNO, whereas profound anemia was almost exclusive to mimicking disorders. CONCLUSION: The EULAR/ACR criteria provide a more sensitive and inclusive framework for CNO while maintaining robust specificity. Their ability to capture atypical cases may reduce diagnostic delay and unnecessary biopsies, though clinical judgment remains essential in distinguishing CNO from overlapping inflammatory arthritis. CLINICAL RELEVANCE: Validation of the EULAR/ACR classification criteria may facilitate earlier and more accurate classification of pediatric CNO and reduce unnecessary invasive diagnostic procedures.