Frailty trajectories in the oldest old: Is the level or the rate of change more predictive of mortality?

BACKGROUND: It is unclear whether frailty index (FI) change captures mortality risk better than and independently of the current FI level, i.e. whether a regular FI assessment among older adults provides additional insights for mortality risk stratification or not. METHODS: We used data from the LASA 75-PLUS-study, which monitored health among 508 older adults (75+) between 2016-2019 every 9 months. Joint models for longitudinal and time-to-event data were used to assess the impact of both current FI and within-person FI change during the last year on mortality risk. RESULTS: 20% of the participants died during 4.5 years of follow-up. Adding within-person FI change to the current FI model improved model fit and it showed that FI increases during the last year were associated with a strong increase in mortality risk. Consequently, the effect of the current FI decreased considerably and became statistically non-significant. CONCLUSIONS: The rate of FI change was more important than the current FI level for short-term mortality prediction among the oldest old, which highlights the benefits of regular frailty assessments.