This study aimed to investigate the association between baseline kidney function and frailty trajectories in middle-aged and older adults.

Data were derived from the China Health and Retirement Longitudinal Study (2011–2018), including 5364 participants aged ≥45 years at baseline with up to four assessment waves over approximately 7 years. Kidney function was evaluated using estimated glomerular filtration rate based on serum creatinine and cystatin C (eGFR _scr-cysc ) and categorized as normal (≥90), mildly reduced (60–89) and moderately-to-severely reduced ( < 60 ml/min/1.73m²). Frailty was assessed using a 30‑item frailty index (0–100 scale). Linear mixed-effects models with random intercepts were used to examine the effects of baseline kidney function and its interaction with time on frailty index trajectories.

At baseline, the mean frailty index was higher in participants with mildly (β=2.28, 95% CI: 1.63–2.94) and moderately-to-severely (β=3.70, 95% CI: 2.41–4.99) reduced kidney function compared to normal kidney function, where β represents the adjusted difference in frailty index relative to the reference group. Frailty index increased over time in all groups; in participants with normal kidney function, it rose by 0.83 points per year (95% CI: 0.77–0.90). The annual increase was 0.26 points greater (95% CI: 0.18–0.35) in the mildly reduced and 0.70 points greater (95% CI: 0.54–0.87) in the moderately-to-severely reduced group. Over approximately 7 years, predicted mean frailty index increased from 15.1 to 20.9, 17.4 to 25.0, and 18.8 to 29.5 in the normal, mildly reduced and moderately-to-severely reduced groups, respectively.

Middle-aged and older adults with lower kidney function exhibited higher frailty index levels at baseline and faster frailty progression over time.