Blood urea nitrogen (BUN) was first used as an index of renal function evaluation, but it cannot meet the requirements of endogenous glomerular filtration rate markers, and is greatly influenced by the disease status of the body, such as congestive heart failure, malnutrition, feeding difficulties, etc.; and more importantly, there is significant passive reabsorption in the renal tubules. Blood creatinine (sCr) has been the main clinical indicator of renal function for more than 40 years, and is popular because of its simplicity and low cost. There are two types of creatinine in the body, exogenous and endogenous, exogenous from dietary sources and endogenous from the metabolism of creatine and creatine phosphate in muscle. Studies have shown that creatinine levels are affected by many factors such as age, gender, body shape, height, muscle mass, and dietary structure, and that renal tubular secretion of creatinine is also a factor that cannot be ignored. Studies have shown that about 30% of patients with chronic kidney disease have a high glomerular filtration rate assessment, and the main influencing factor is the secretion of creatinine by the renal tubules; if drugs inhibit glomerular secretion, strenuous physical activity and eating non-vegetarian food, the glomerular filtration rate is underestimated; in addition, the sCr will increase only when the glomerular filtration rate of the body falls below 30% of the normal level. The above data suggest that sCr is not an accurate and reliable marker of renal function, prompting researchers to seek new and more reliable, easy and fast markers of glomerular filtration rate. Cystatin C is a small, positively charged molecule that is freely filtered from the glomerulus and is completely reabsorbed by the tubular epithelium and degraded intracellularly, without entering the periglomerular circulation. The tubular epithelium also does not secrete cystatin C into the tubular lumen. Based on these characteristics, cystatin C is by far the best marker of the ideal endogenous glomerular filtration rate. Urinary cystatin C levels are very low and cannot be used to judge renal function by per-cystatin C. Serum cystatin C levels can directly reflect renal function.