Cystatin C (CysC) is a cysteine protease present in the lysosomes of nucleated cells. It is produced at a constant rate and is freely filtered by the glomerulus. It is not secreted by the kidney and exhibits a similar relationship to GFR as creatinine. Diet and muscle mass have less effect on its concentration, but other factors such as age, sex, medications, and disease may influence its concentration (Clarke, 2016).
CysC is not measured in the urine as it is fully resorbed by the renal tubules. However, it is measured in plasma using standardized immunoassays and is useful for determining the eGFR.
The NKF-ASN Task Force has recommended the national adoption of cystatin C testing to assess GFR in patients with abnormally high or low muscle mass.
