Cystatin C is a commonly used indicator for determining early kidney injury, and it is also a product of cystine metabolism in the body. Cystatin C is mainly secreted by glomeruli, not secreted or recycled by tubules, so it can reflect the glomerular filtration function to a certain extent, i.e. glomerular filtration rate. It is generally believed that this indicator is more sensitive than creatinine and urea nitrogen, and can detect abnormalities at the early stage of renal disease, so if serum cystatin C is elevated, while creatinine and urea nitrogen are normal, it often suggests early kidney injury. Glucocorticoids, thyroid function, tumors, obesity, diabetes, inflammation, and positive fluid balance all have an impact on cystatin C. Patients with abnormal cystatin C are recommended to consult regular hospitals and standardize the examination and treatment under the guidance of physicians.