A cysteine proteinase inhibitor found in the bloodstream in elevated concentrations in patients with impaired kidney function. It is a small protein composed of 120 amino acids (mass 13 kD) that is produced by nucleated cells throughout the body and easily filtered by the glomeruli of the kidneys. It is reabsorbed and catabolized by proximal tubular cells. Because levels of cystatin C do not depend on a patient’s age, height, muscle mass, or weight, it is thought to be a better measure of kidney function than the creatinine clearance test, which is most often used to assess renal health.