The estimated glomerular filtration rate (eGFR) is of pivotal importance to staging, risk stratification, drug administration, and clinical decision in patients with chronic kidney disease (CKD).1 In clinical practice, serum creatinine is routinely used for eGFR. However, creatinine is related to age, sex, and muscle mass and current guidance recommends additionally measuring cystatin C for confirmatory testing, especially in subjects with wasting or large muscle mass.2 The decrease in muscle mass observed in aging, CKD, and many catabolic conditions leads to decreased serum creatinine levels and can also cause an overestimation of the creatinine-based eGFR (eGFRcr), while cystatin C-based eGFR (eGFRcys) provides a more accurate estimation of kidney function.