Current practice guidelines for CKD recommend measuring cystatin C when precision of eGFR is required and suggest adding cystatin C for confirmatory testing in situations when creatinine is less accurate, such as in persons who may underproduce creatinine because they are frail.5,6,17 As efforts advance to improve CKD detection and monitoring and to eliminate the use of race in estimating kidney function, cystatin C will be increasingly incorporated into routine clinical care, and clinicians will commonly encounter persons with large eGFRdiffcys-cr. This evidence concerns the gene CST3 and chronic kidney disease.