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 mGFR and TTMLA in the AGES-Reykjavik Study.

Exposure: MGFcr, the product of serum creatinine (Scr) and mGFR obtained using plasma iohexol clearance and eGFcr, the product of Scr and estimated GFR using serum cystatin (eGFRcys).

Outcomes: TTMLA 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 (SD) age and mGFR were 80.3 (4.0) years and 62.3 (16.5) mL/min/1.73 m2, respectively. The lowest sex-specific tertile of mGFcr, compared to the highest tertile, was associated with a 14.6 (95% CI, 11.5-17.6) cm2/1.73 m2 lower TTMLA in men, and a 7.9 (95% CI, 5.5-10.2) cm2/1.73 m2 lower TTMLA in women. Significant associations were observed between eGFcr and TTMLA. Correlations of eGFcr with TTMLA 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.