Where does frailty collide?

In survival analysis a frailty is an unobserved random effect that scales an individual's or a group's hazard up or down, standing in for the heterogeneity in susceptibility that measured covariates miss; a shared frailty lets correlated units, such as littermates or patients in one clinic, carry the same multiplier. Vaupel, Manton, and Stallard's 1979 The impact of heterogeneity in individual frailty on the dynamics of mortality introduced the term for this latent multiplicative factor and showed how ignoring it bends the observed mortality curve. The referent is a statistical latent variable on the hazard, estimated rather than measured, and a different construct from the clinical frailty of an aging body.[1]

Unit

The unobserved multiplicative factor on an individual's or group's hazard

Recovery means

Not applicable

Measured by

Estimated as a random-effect variance in a frailty (random-effects) survival model, not observed directly

In geriatric medicine frailty is a clinical syndrome of diminished physiological reserve across multiple body systems that leaves an older person vulnerable to a disproportionate decline after even a minor stressor. Fried and colleagues' 2001 Frailty in older adults: Evidence for a phenotype made it a measurable phenotype (unintentional weight loss, self-reported exhaustion, weakness, slow walking speed, and low physical activity), with a person counting as frail when enough of these are present. The referent is an observable bodily state in a patient, assessed directly, unrelated to a latent term in a hazard model beyond the shared name.[2]

Unit

An older person's degree of physiological vulnerability to stressors

Recovery means

Partly reversible: exercise and nutrition can move a person back toward robustness, so frailty marks vulnerability rather than a fixed end state

Measured by

Assessed with the five-item Fried phenotype or with cumulative-deficit frailty indices