Cystatin C usually refers to serum cystatin C. High levels of cystatin C are generally associated with renal impairment and are usually seen in diabetic nephropathy, hypertensive nephropathy and glomerulonephritis. Cystatin C is a low molecular weight non-glycosylated basic protein, which is an ideal endogenous marker of glomerular filtration rate due to its small relative molecular mass, its ability to freely pass through the glomerular filtration membrane, and its almost complete reabsorption and degradation in the proximal tubule without re-entry into the circulation, and no secretion by the tubule. The concentration of cystatin C in serum correlates well with the glomerular filtration rate and can accurately reflect the glomerular filtration rate, especially for evaluating early renal impairment. If the serum cystatin C level is elevated, it usually suggests that the patient’s glomerular filtration rate is reduced, indicating that the patient has renal impairment, which is usually common in diabetic nephropathy, hypertensive nephropathy and glomerulonephritis. It is recommended that patients with high cystatin C levels should consult a doctor in time for further relevant examinations to clarify the diagnosis and follow the doctor’s instructions for treatment.