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 (GF) reflects muscle mass, and previously proposed estimated GF (eGF), as a practical index of muscle mass in older adults. This study investigated whether measured GF (mGF) and eGF 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 TTMLA in the AGES-Reykjavik Study.

EXPOSURE

Measured GF, the product of serum creatinine (Scr) and mGFR obtained using plasma iohexol clearance and eGF, the product of Scr and estimated glomerular filtration rate using serum cystatin C (Scys).

OUTCOME

TTMLA measured using computed tomography.

ANALYTICAL APPROACH

Sex-specific Pearson's correlation and linear regression analyses using continuous and categorical mGF and eGF. 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/1.73m, respectively. The lowest sex-specific tertile of mGF, compared with the highest tertile, was associated with a 14.6 (95% CI, 11.5-17.6) cm/1.73m lower TTMLA in men, and a 7.9 (95% CI, 5.5-10.2) cm/1.73m lower TTMLA in women. Significant associations were observed between eGF and TTMLA. Correlations of eGF 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 GF as an index of muscle mass among older adults and the use of eGF as a practical alternative to mGF in the clinical setting.

PLAIN-LANGUAGE SUMMARY: Low muscle mass is common among older adults and is associated with poor clinical outcomes. Quantifying muscle mass is challenging in routine clinical practice. We evaluated whether glomerular filtration of creatinine (GF) could serve as an index of muscle mass. We performed a cross-sectional study including 794 older adults who underwent computed tomography for thigh muscle lean area as a directly measured indicator of total body muscle mass. Significant positive associations between thigh muscle lean area and both measured GF (serum creatinine [Scr] ×measured glomerular filtration rate [GFR]) and estimated GF (Scr ×estimated GFR based on serum cystatin C [Scys]), a more practical index, were shown. These findings suggest the value of using eGF, a simply obtained novel index in the clinical setting, to assess muscle mass among older adults.