Cystatin C–based estimated glomerular filtration rate (eGFRcystatin C) has been routinely used clinically in Sweden since 1994 to monitor kidney disease.1 A strategy to best estimate glomerular filtration rate (GFR) by simultaneously analyzing eGFRcystatin C and creatinine-based estimated GFR (eGFRcreatinine) and then comparing these estimates has been used since 2010.2 In doing so, a large repository of data has been accumulated, revealing discrepancies between eGFRcystatin C and eGFRcreatinine.