Cystatin C: A serum protein that is filtered out of the blood by the kidneys and that serves as a measure of kidney function. Cystatin C is produced steadily by all types of nucleated cells in the body. Its low molecular mass allows it to be freely filtered by the glomerular membrane in the kidney.

Its concentration in blood correlates with the glomerular filtration rate. The levels of cystatin C are independent of weight and height, muscle mass, age (over a year of age), and sex. Measurements can be made and interpreted from a single random sample. Cystatin C is a better marker of the glomerular filtration rate and hence of kidney function than creatinine which was the most commonly used measure of kidney function.