Differences in creatinine and cystatin C-based estimates of glomerular filtration rate (eGFRDiff = eGFRCr – eGFRCysC) may reflect differences in muscle mass. We sought to determine if eGFRDiff (1) reflects lean mass, (2) identifies sarcopenic individuals beyond estimates based on age, body mass index (BMI), and sex; and (3) demonstrates associations differently in those with and without chronic kidney disease (CKD).