Background: The Rib Fracture Frailty (RFF) Index is an internally validated machine learning-based risk assessment tool for adult patients with rib fractures that requires minimal provider entry. Existing frailty risk scores have yet to undergo head-to-head performance comparison with age, a widely used proxy for frailty in clinical practice. Our aim was to externally validate the RFF Index in a small-scale implementation feasibility study.

Methods: Prospective observational cohort study conducted across five ACS COT-verified trauma centers. Participants included ≥18-year-old adults presenting January 1, 2021, to December 31, 2021, with traumatic rib fractures. The primary outcome was a composite outcome score comprised of three clinical factors: hospitalization ≥5 days, discharge disposition, and inpatient mortality. Proportional odds logistic regression evaluated associations of age model or RFF Index score model with composite outcome scores. Models were compared using standard discrimination and calibration metrics. Secondary analysis delineated predictive performance among patients with lower (Injury Severity Score < 15) and higher Injury Severity Score ≥ 15) injury burden.

Results: Of 849 participants, 546 (64%) were male and median age was 62 years (interquartile range, 46-76 years). A one-point increase in RFF score was associated with 6% increased odds of higher composite outcome score (odds ratio [OR], 1.06; 95% confidence interval [95% CI], 1.04-1.08), while a 1-year increase in age did not show statistically significant association (OR, 1.10; 95% CI, 0.75-1.61). The RFF score had higher discrimination (OR, 0.09; 95% CI, 0.08-0.11 vs. OR, 0.06; 95% CI, 0.04-0.08; p = 0.04) and calibration performance compared with age, but on secondary analysis, higher predictive performance was limited to patients with lower injury burden. Both RFF Index and age had poor calibration for predicting patients discharged to home after hospitalization ≥5 days.

Conclusion: This prospective external validation study found RFF Index may be a better alternative to age for predicting adverse outcomes among patients with traumatic rib fractures and lower overall injury burden. Staged implementation studies in accordance with clinical prediction model implementation guidelines are required to evaluate the RFF Index's clinical efficacy and guide potential adoption. LEVEL OF EVIDENCE: Prognostic and Epidemiological; Level II.