Simple Bedside Muscle‐Related Indicators Improve the Identification of Reduced Kidney Function in Hospitalized Older Adults: A Multicenter Cross‐Sectional Study

ABSTRACT Aim Creatinine‐based estimated glomerular filtration rate (eGFRcr) may overestimate kidney function in older adults with reduced muscle mass. Although current guidelines recommend cystatin C when creatinine‐based estimates are unreliable, its routine use remains limited. We evaluated whether incorporating simple bedside muscle‐related indicators into eGFRcr improves the identification of reduced kidney function defined by cystatin C‐based estimated glomerular filtration rate (eGFRcys). Methods We conducted a multicenter cross‐sectional study of hospitalized adults aged ≥ 60 years at four Japanese community hospitals. Three models were evaluated: eGFRcr only (Basic model), eGFRcr plus calf circumference (Model 1), and eGFRcr plus the finger‐ring test (Model 2). The primary outcome was continuous eGFRcys, and eGFRcys < 45 mL/min/1.73 m 2 . Results Among 98 participants (median age 79 years), adding muscle‐related indicators modestly improved model performance. Adjusted R 2 increased from 0.641 (Basic model) to 0.668 (Model 1) and 0.680 (Model 2). The proportion of participants correctly classified with eGFRcys < 45 mL/min/1.73 m 2 increased from 56.1% with the eGFRcr alone to 80.5% with both extended models. Discrimination also improved (area under the curve: 0.882 vs. 0.921 and 0.918, respectively), although the differences were not statistically significant. Conclusion Incorporating simple bedside muscle‐related indicators to creatinine‐based assessment improved the identification of reduced kidney function defined by eGFRcys in hospitalized older adults. These measures may complement eGFRcr interpretation when cystatin C measurement is unavailable or delayed. External validation is required before clinical implementation.