Age-Adjusted Modified Frailty Index: Association With Hip Fracture Discharge Functional Dependence

Introduction: Hip fractures are a common traumatic injury that carry significant morbidity and mortality, and prognostication of functional outcome is becoming increasingly salient. Across multiple surgical specialties, the five-item and 11-item Modified Frailty Index (mFI-5 and mFI-11) have been found to be convenient, quick, and sensitive tools for identifying patients at risk for perioperative complications. A prior study described the superiority of an Age-Adjusted Modified Frailty Index (aamFI) for predicting perioperative complications compared to the mFI-5 in an elective hip surgery. We sought to externally validate the aamFI in a multicenter hip fracture cohort and hypothesize that these risk scores would not only predict functional dependence (FD) at discharge, but that the aamFI would outperform the mFI-5 and mFI-11.

Methods: The Pennsylvania Trauma Systems Foundation registry was queried from 2010 to 2020 for CPT codes, ICD-9 and ICD-10 codes pertaining to hip fracture patients. Patients with missing locomotion and transfer mobility data were excluded. FD status was determined by discharge locomotion and transfer mobility scores per existing methodology. Univariable and Multivariable analysis as well as receiver operator characteristic curves were used to evaluate and compare the three indices for prediction of functional status at discharge. P value < 0.05 was considered significant.

Results: Twelve thousand seven hundred and forty patients met inclusion criteria (FD: 8183; functional independent 4557). On univariable logistic regression analysis, the mFI-11 (odds ratio [OR] 1.52, 95% confidence interval [CI] 1.47-1.58, P < 0.05), mFI-5 (OR 1.57, 95% CI 1.51-1.63), and aamFI (OR 1.57, 95% CI 1.52-1.62, P < 0.05) were associated with FD. On multivariable logistic regression analysis for predictors of FD, when controlling for age (for the mFI-11 and mFI-5), sex, injury severity score, and admission vitals (systolic blood pressure and respiratory rate), higher mFI-11 and mFI-5 scores independently predicted FD at discharge (OR 1.23, 95% CI 1.18-1.28, P < 0.05 and OR 1.23, 95% CI 1.18-1.29P < 0.05 respectively). Higher aaMFI scores had superior association with functional dependence (OR 1.59, 95% CI 1.54-1.64, P < 0.05). Receiver operator characteristic curves for the mFI-11, mFI-5, and aaMFI showed comparable diagnostic strength (area under curve [AUC] = 0.63 95% CI 0.62-0.64, P < 0.05; AUC = 0.63 95% CI 0.62-0.64, P < 0.05; and AUC = 0.67 95% CI 0.65-0.67, P < 0.05 respectively).

Conclusions: The mFI-11, mFI-5, and aamFI are predictive of functional outcome following hip fracture. By including age, the aamFI retains the ease of use of the mFI-5 while improving its prognostic utility for functional outcome.