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Background The International Classification of Diseases, 10th Revision, Clinical Modification (ICD-10-CM) code S09.90 (‘unspecified injury of head’) is frequently used in emergency departments (EDs) but excluded from the Centers for Disease Control and Prevention (CDC)’s standard traumatic brain injury (TBI) surveillance definition due to its non-specific nature. This study identifies patient-level and ED-level factors associated with the use of S09.90 and examines whether the relationship between age and S09.90 coding varies by injury severity.Methods A retrospective analysis of 2022–2023 TBI-related ED visits in North Carolina (NC) was conducted using data from the NC Disease Event Tracking and Epidemiologic Collection Tool (NC DETECT). TBI-related ED visits were identified based on ICD-10-CM codes, including those with and without S09.90. Injury severity was defined using patient disposition. Bivariate and multivariable logistic regressions identified predictors of S09.90 coding, and an interaction term between age and injury severity assessed modification.Results Among 229 516 TBI-related ED visits, 80.5% (n=1 84 671) were coded with S09.90. S09.90 coding was more likely among older adults (65+years; adjusted OR (aOR)=1.68, 95% CI 1.57 to 1.79) and children (0–14 years; aOR=1.60, 95% CI 1.50 to 1.70) compared with working-age adults, but only among non-severe injuries. For severe injuries, children were less likely to be coded with S09.90 (aOR=0.79, 95% CI 0.68 to 0.91). S09.90 use was also associated with female patients and absence of comorbidities.Discussion and conclusions Findings underscore the need for improved diagnostic specificity and consideration of S09.90 use in TBI surveillance to avoid underestimating TBI burden in specific populations.