Frailty (clinical syndrome and frailty index)

Frailty is a clinical state of increased vulnerability to stressors resulting from accumulated deficits across multiple physiological systems, leading to diminished reserve and resilience. Two complementary operationalisations dominate the literature: the phenotypic frailty model of Fried and colleagues (2001, Cardiovascular Health Study), which defines frailty by at least three of five criteria (unintentional weight loss, exhaustion, low grip strength, slow walking speed, low physical activity); and the Frailty Index of Mitnitski and Rockwood, which counts the proportion of health deficits present across 30–70 items (symptoms, signs, diagnoses, laboratory values). Both predict adverse outcomes—falls, hospitalisation, disability, and mortality—independently of chronological age, and frailty prevalence rises sharply after age 80. In geroscience, it is a key functional outcome for evaluating senolytic, senostatic, and other geroprotective interventions.