Cystatin C (CysC) is a low molecular weight, non-glycosylated basic protein composed of 120 amino acids, with a molecular weight of approximately 13 kDa. It is a member of the cysteine protease inhibitor superfamily and is produced at a constant rate by all nucleated cells in the body. As an endogenous marker, CysC is freely filtered by the glomeruli, reabsorbed and metabolized in the proximal tubules, and does not return to the bloodstream. Its serum concentration is independent of factors such as age, gender, diet, and muscle mass, making it an ideal indicator for assessing glomerular filtration rate (GFR) and early kidney function impairment. Elevated levels of CysC may indicate decreased kidney function, while its stability and sensitivity make it superior to traditional markers like serum creatinine.