Cystatin C

The Cystatin C (CysC) test is used to establish baseline renal function and to monitor changes over time by estimating glomerular filtration rate from serum CysC. It is particularly useful when creatinine-based eGFR may be unreliable because of atypical muscle mass, pediatric or older age, or dietary factors such as a vegetarian diet. In the setting of chronic kidney disease, cystatin C may also contribute to cardiovascular risk stratification.

Cystatin C is produced at a relatively constant rate by nucleated cells. It passes freely through the glomerular filter and is then almost completely reabsorbed and degraded by proximal tubular cells without returning to the circulation, so serum levels primarily reflect glomerular filtration. When GFR declines, serum cystatin C increases, and validated equations enable eGFR calculation from cystatin C measurements. Beyond kidney assessment, CysC participates in extracellular matrix turnover and has been associated with atherosclerosis and myocardial remodeling; its application in cardiovascular risk assessment remains under active study. During normal pregnancy, eGFR estimated from cystatin C may be less accurate, and thyroid dysfunction can modify serum cystatin C independently of GFR.