Cys C is a cysteine protease inhibitor, also known as γ- Microprotein and γ- Post globulin, widely present in nucleated cells and body fluids of various tissues, is a low molecular weight, alkaline non glycosylated protein with a molecular weight of 13.3KD. It is composed of 122 amino acid residues and can be produced by all nucleated cells in the body with a constant production rate. Cystatin C in circulation is only cleared by glomerular filtration and is an endogenous marker that reflects changes in glomerular filtration rate. It is reabsorbed in the proximal tubules, but is completely metabolized and decomposed after reabsorption and does not return to the blood. Therefore, its blood concentration is determined by glomerular filtration and does not depend on any external factors such as gender, age, or diet. It is an ideal homologous marker that reflects changes in glomerular filtration rate.