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. Under normal circumstances, the concentration of Cys C in serum and plasma is 0.51-1.09mg/L (reference range). When renal function is impaired, the concentration of Cys C in the blood changes with changes in glomerular filtration rate. When kidney failure occurs, the glomerular filtration rate decreases, and the concentration of Cys C in the blood can increase by more than 10 times. If the glomerular filtration rate is normal but the renal tubules are malfunctioning, it will hinder the absorption and rapid decomposition of Cys C in the renal tubules, increasing the concentration in urine by more than 100 times.