In their editorial, Grubb et al examine differences between cystatin C– and creatinine-based estimated glomerular filtration rate (eGFRcys and eGFRcr, respectively). An eGFRcys:eGFRcr ratio <0.7 occurs in 0.3%-36% of patients (depending on the population) and is associated with worse health outcomes. The authors demonstrate how this ratio can be influenced both by nonrenal factors and by selective hypofiltration of cystatin C. However, most countries do not routinely measure cystatin C, limiting the application of this important distinction.