The correct evaluation of renal function is crucial in clinical practice for diagnosis (e.g., imaging with contrast agents), treatment (e.g., drug dosing) and prognostic assessment (e.g., clinical decision making). For this purpose, an estimate of glomerular filtration rate (eGFR) is usually provided by formulas based on serum creatinine [1–3]. Notably, serum creatinine is affected by factors other than GFR: as a breakdown product of muscle metabolism, its levels decline with decreasing muscle mass, such as in age-dependent sarcopenia.
