Cystatin C is increasingly used to estimate glomerular filtration rate (eGFRcys) in settings where estimated GFR 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.