In addition, the difference between eGFRcys and eGFRcr, namely, eGFRdiff, also has clinical implications among the aging population, and uncertainty remains regarding the prospective association of eGFRdiff with long-term frailty trajectories.10 Serum creatinine levels are more likely to be altered by muscle mass than by cystatin C levels, and higher eGFRcr (ie, lower creatinine) may indicate low muscle mass.8 Therefore, the negative eGFRdiff may be explained in part by sarcopenia, which is associated with a variety of negative outcomes in older adults.12 Here, CST3 is linked to sarcopenia.