Short-term Frailty Index Fluctuations in Older Adults: Noise or Signal?

Reversible short-term fluctuations in the frailty index (FI) are often thought of as representing only noise or error. Here, we assess (i) the size and source of short-term FI fluctuations, (ii) variation across sociodemographic characteristics, (iii) association with chronic diseases, (iv) correlation with age, frailty level, frailty change, and mortality, and (v) whether fluctuations reflect discrete health transitions.

Nationwide, biweekly longitudinal data from 426 community-dwelling older adults (70+) were collected in the FRequent health Assessment In Later life (FRAIL70+) study using a measurement burst design (5 122 repeated observations, median of 13 repeated observations per person). We calculated the intraindividual standard deviation of the FI and used location-scale mixed regression models.

Mean intraindividual standard deviation was 0.04 (standard deviation = .03). Fluctuations were driven foremost by cognitive problems, somatic symptoms, and limitations in instrumental and mobility-related activities of daily living. Short-term fluctuations correlated with higher FI levels (r = 0.62), 1-year FI change (r = 0.26), and older age (+3% per year). Older adults who took to bed due to a health problem (+50%), those who had an overnight hospital stay (+50%), and those who died during follow-up (+44%) exhibited more FI fluctuations.

Short-term FI fluctuations were neither small nor random. Instead, as older adults become frailer, their measured health also becomes more unstable; hence, short-term fluctuations in overall health status can be seen as a concomitant phenomenon of the aging process. Researchers and clinicians should be aware of the existence of reversible fluctuations in the FI over weeks and months and its consequences for frailty monitoring.