Glomerular Filtration of Creatinine: Validation of a Novel Index of Muscle Mass Among Older Adults

Rationale & Objective: Low muscle mass is common among older adults and associated with poor prognosis. Quantifying muscle mass is challenging in routine clinical practice. We hypothesized that glomerular filtration of creatinine (GFcr) reflects muscle mass, and previously proposed estimated GFcr (eGFcr), as a practical index of muscle mass in older adults. This study investigated whether measured GFcr (mGFcr) and eGFcr are similarly associated with the direct measure of muscle mass, the thigh total muscle lean area (TTMLA). Study Design: Cross-sectional analysis of a community-based prospective cohort. Setting & Participants: A total of 794 older adults with measured glomerular filtration rate (mGFR) and TTM LA in the AGES-Reykjavik Study. Exposure: Measured GFcr, the product of serum creatinine (Scr) and mGFR obtained using plasma iohexol clearance and eG Fcr, the product of Scr and estimated glomerular filtration rate using serum cystatin C (Scys). Outcome: TTM LA measured using computed tomography. Analytical Approach: Sex-specific Pearson's correlation and linear regression analyses using continuous and categorical mGFcr and eGFcr. Covariates included demographic, behavioral, and clinical variables, and comorbid conditions. Results: The mean age and mGFR were 80.3 +/- 4.0 (SD) years and 62.3 +/- 16.5 (SD) mL/ min/1.73 m2, respectively. The lowest sex-specific tertile of mGFcr, compared with the highest tertile, was associated with a 14.6 (95% CI, 11.5-17.6) cm2/1.73 m2 lower TTM LA in men, and a 7.9 (95% CI, 5.5-10.2) cm2/1.73 m2 lower TTM LA in women. Significant associations were observed between eGFcr and TTM LA. Correlations of eGFcr with TTM LA were generally as strong or stronger than correlations of alternative indices derived from Scr and Scys. Limitations: Residual confounding by measured and unmeasured variables. Conclusions: These findings support the validity of GFcr as an index of muscle mass among older adults and the use of eGFcr as a practical alternative to mGFcr in the clinical setting.