Cystatin C is increasingly used to estimate glomerular filtration rate (eGFRcys) in settings where eGFR from creatinine (eGFRcr) may be less accurate.1 Optimal use of any eGFR equation requires understanding of the factors that influence the marker independent of changes in kidney filtration. Non-GFR determinants of cystatin C are less well understood than for creatinine. Prior studies suggest that cystatin C is generated in high cell turnover states such as inflammation, hyperthyroidism, and cancer.