BACKGROUND: Frailty is the age-related decline in functional reserve, resulting in physical and disease vulnerability. The established 5-Factor Modified Frailty Index (mFI-5), based on the National Surgical Quality Improvement Program (NSQIP) database, is predictive but does not encompass core components of frailty - namely physical abilities, cognitive status, and nutrition. Thus, we sought to establish a scoring index more indicative of the frailty phenotype. STUDY DESIGN: Our retrospective study included patients ≥ 75 years of age, from NSQIP 2021-2022. The Modified 4-Factor Functional Frailty Index (mFF-4) is calculated by summing each present variable: prior falls, dementia, low body mass index, and non-independent functional status. Patients were stratified into frailty levels: none (mFF-4=0), intermediate (mFF-4=1), and high (mFF-4=2+). Area under the curve (AUC) and Akaike information criterion (AIC) values assessed predictability relative to the reference mFI-5. Odds ratios for 30-day outcomes were calculated using logistic regression. RESULTS: The sample included n=265,041. Median age was 79 (interquartile range 77-84). The mFF-4 AUCs were consistently higher than mFI-5
mFF-4=2+ AUCs were 0.8679 for mortality, 0.7298 for postoperative complications, and 0.7645 for geriatric outcomes. Delta(AICs) values exceeded 2, favoring mFF-4 models. Compared to non-frail, mFF-4=2+ had increased odds of mortality (odds ratio 2.47, confidence interval 2.31-2.63), postoperative complications (1.37, 1.32-1.43), and geriatric outcomes (4.54, 4.30-4.79). CONCLUSIONS: The new mFF-4 scoring tool embodies the true frailty phenotype to accurately predict 30-day outcomes across surgical subspecialties and the overall population. It can serve as a new NSQIP "frailty" definition, while also providing a framework for surgeons and patients alike to conceptualize the elevated risks it poses.