Frailty Assessment in an Aging Mouse Model

A frailty index (FI) is a powerful method of assessing animal models of aging, serving as a proxy measure of biologic age and health span. Frailty represents a state of increased vulnerability to adverse outcomes due to the accumulation of health deficits that limit independence. The concept is valuable for animal studies that inform human geroscience. For example, using FI scores, animals with a similar chronological age can be differentiated by their biological age. Thus, FI can be used in preclinical studies to measure biological age, track frailty longitudinally, and assess health span. A murine frailty index was recently developed using 31 health-related deficits. However, its length and requirement for specialized equipment pose limitations in practical use. Here, we present an adapted 16 item frailty index that excludes parameters requiring special instrumentation or invasive testing. The selected deficits are easily observable or measurable and evaluate integument, musculoskeletal, neuromuscular, sensory, urogenital, and respiratory systems. Body weight is measured for longitudinal tracking but excluded from the FI calculation. Each deficit is scored as 0, 0.5, or 1, based on severity, with the FI computed as the mean of the 16 deficit scores. This streamlined FI allows rapid assessment of large cohorts, can be applied longitudinally to the same animals, and requires no specialized equipment. We demonstrate its feasibility in C57BL/6×129 mice, showing age-associated increases in frailty, variation among individual mice, and the ability to detect deficits across multiple systems. This tool enables laboratories to incorporate frailty into diverse experimental designs, provides a practical approach for assessing health span, and enhances translational relevance in geroscience. This frailty index is adapted from previously validated mouse clinical frailty methodologies and is intended for straightforward, longitudinal use alongside ongoing studies.