Glomerular filtration rate estimated using creatinine level (eGFRcr) tends to be high in people with low muscle mass, whereas that based on cystatin C level (eGFRcys) is not as affected by muscle mass.1 It is not uncommon to see patients with discrepancies between the 2 eGFRs. We hypothesized that prognostic information is embedded in the difference in these markers; namely, that frailty would be less prevalent in older adults with eGFRcys>eGFRcr, and that these individuals would be at lower risk for incident frailty and mortality than those having a minimal difference between the 2 estimates.