Comparing frailty index distribution, mortality and frailty dynamics across multiple countries: applying the 'zero-state' principle to harmonised ageing data.

Abstract

Introduction: Frailty is a dynamic state linked to disability, hospitalisation and mortality. Cross-country comparisons of frailty progression remain limited. We compared frailty distribution, mortality risk and deficit accumulation across 17 middle-/high-income countries. We then examined whether outcomes among individuals with minimal baseline deficits ('zero-state') reflect broader population health patterns to explore utility of the 'zero-state' as a marker of population health.

Methods: Data were from five harmonised longitudinal ageing surveys (Health and Retirement Survey, Survey of Health, Ageing and Retirement in Europe, English Longitudinal Study of Ageing, Chinese Health and Retirement Longitudinal Survey, Mexican Health and Ageing Study). A 31-item frailty index (FI) was constructed using identical variables at baseline, 2- and 4-year follow-up. Participants with <2 deficits were classified as in 'zero-state'. Outcomes: all-cause mortality and FI change among survivors. Logistic regression (mortality) and negative binomial models (deficit accumulation) were adjusted for age, sex and baseline FI. Analyses were stratified by zero-state and non-zero-state.

Results: Baseline frailty varied between countries: highest in the USA, Mexico and Southern/Eastern European nations, and lowest in Northern/Central Europe. Mortality and deficit accumulation differed substantially. Mexico showed highest mortality and fastest accumulation, while Switzerland and Scandinavian countries showed the lowest. Zero-state analyses revealed larger between-country differences, but lower statistical power, than analyses of those with higher FI. Some countries (e.g. USA, UK) showed favourable zero-state outcomes but more rapid progression among individuals with higher baseline FI.

Conclusion: Frailty distribution, mortality and frailty progression vary widely across settings, with poorer outcomes in lower-income countries. Zero-state trajectories capture important population differences but do not fully reflect patterns among those with existing frailty, potentially reflecting within-country inequalities.