Serum cystatin C greater than 2.5mg/L is considered abnormal and usually suggests kidney damage, and its normal value ranges from about 0.6 to 2.5mg/L, which varies from hospital to hospital and from instrument to instrument. Serum cystatin C is cysteine protease inhibitor C, which is a positively charged protein with good early assessment and diagnostic value for renal lesions. When early impairment of renal function occurs in various renal diseases such as diabetic nephropathy, hypertensive nephropathy, lupus nephritis and others, serum urea nitrogen and blood creatinine may be in the normal range, but serum cystatin C may already be elevated. In addition, serum cystatin C can determine the changes in renal function, such as acute kidney injury, if the serum cystatin C concentration decreases, indicating that renal function is also gradually improving. Serum cystatin C can reflect early kidney injury earlier than blood creatinine and urea nitrogen, therefore, when abnormalities occur, it is recommended to go to the hospital, under the guidance of the doctor to find the cause of the disease, and timely treatment, so as to avoid delaying the condition.