Cardiorenal syndrome type 2 (CRS2) is characterized by chronic heart failure leading to chronic kidney disease (CKD).1 In these patients, kidney function needs to be evaluated for therapeutic and prognostic purposes. Because patients with CRS2 have cardiac cachexia from anorexia and increased catabolism,2 creatinine-based eGFR equations may be subject to important errors.3 Cystatin C, which unlike creatinine is independent of lean body mass, can also be used to estimate kidney function.4 Cystatin C has been reported to be a biomarker of early declines in kidney function and a prognostic marker.