Glomerular filtration rate is estimated either based on creatinine (eGFRCr), which tends to be high in people with low muscle mass, or cystatin C (eGFRCys), which is not as affected by muscle mass.1 It is not uncommon to see clinic patients with discrepancies between the two. We hypothesized that prognostic information is embedded in the difference in these markers. We propose that older adults with eGFRCys > eGFRCr will have less prevalent frailty, and will be at lower risk for incident frailty and mortality, compared with those having a minimal difference between the two estimates.