Kidney Function Estimated From Cystatin C, But Not Creatinine, Is Related to Objective Tests of Physical Performance in Community-Dwelling Older Adults.

BACKGROUND: The burden of chronic kidney disease is highest among older adults but the significance of a diminished level of kidney function in this heterogeneous population is poorly understood. We sought to examine the relationship between estimated glomerular filtration rate (eGFR) and objective physical performance in older adults. METHODS: Cross-sectional analysis of 4,562 participants from The Irish Longitudinal Study on Ageing, a national cohort of community-dwelling adults aged ≥50 years. We used multivariable linear or quantile regression to model the association between categories of cystatin C (eGFRcys) or creatinine eGFR (eGFRcr) and the following outcomes: gait speed, timed-up-and-go (TUG) and grip strength. Relationships were further explored using natural eGFR splines. We examined effect modification by age in the relationship between eGFR and gait speed. RESULTS: Mean (SD) age was 61.8 (8.3) years, 53.6% were female and median (IQR) eGFRcys was 82 (70-94) mL/min/1.73m2. In multivariable-adjusted models, participants in the lowest eGFRcys category (< 45 mL/min/1.73m2) had 3.32 cm/s (95% confidence interval [95%CI] 0.02-6.62) slower mean gait speed, 1.32 kg (95%CI 0.20-2.44) lower mean grip strength, and 0.31 seconds (95% CI -0.04 to 0.65) longer median TUG versus the reference group (eGFRcys ≥ 90 mL/min/1.73m2). The relationship between eGFRcys and outcomes appeared linear but varied by age. The association between eGFRcr and outcomes tended towards a U-shape. CONCLUSIONS: Cystatin C eGFR was linearly related to poorer physical performance beyond middle age among community-dwelling adults. The non-linear relationships observed with eGFRcr underscore the limitations of creatinine as a predictor of frailty outcomes in older individuals.