Admission albumin and C-reactive protein-to-lymphocyte ratio are associated with MRI-defined abscess in children with acute hematogenous osteomyelitis: a two-center retrospective study.

Introduction: Intramedullary and subperiosteal abscesses are clinically relevant MRI-defined phenotypes in children with acute hematogenous osteomyelitis (AHO). This study examined admission-phase markers associated with MRI-defined abscess and constructed an exploratory pre-MRI risk-stratification model. Methods: This two-center retrospective cohort study included 105 children with AHO who underwent MRI within 72 hours of admission. Clinical variables and admission laboratory indices were analyzed using univariable analyses and multivariable Firth penalized logistic regression. Model performance was assessed using receiver operating characteristic analysis, calibration assessment, decision curve analysis, stratified cross-validation, and bootstrap optimism correction. Results: MRI-defined abscess was present in 45 children (42.9%). Lower albumin and higher CRP-to-lymphocyte ratio were independently associated with MRI-defined abscess, whereas fever at admission was retained as a prespecified clinically relevant covariate rather than interpreted as an independent predictor. The final model showed modest discrimination (apparent AUC, 0.711; bootstrap 95% CI, 0.624-0.826; optimism-corrected AUC, 0.689). At the exploratory Youden-derived cutoff of 0.456, sensitivity was 0.600 and specificity was 0.767. Conclusion: Admission albumin and CRP-to-lymphocyte ratio are readily available markers associated with MRI-defined abscess and may support early identification of children who warrant heightened attention to timely MRI and multidisciplinary review. The model requires external validation and should complement, not replace or delay, clinical assessment, MRI, and surgical evaluation.