Glomerular filtration function is one of the most important functions of the kidney and is generally expressed as glomerular filtration rate. Because there is no way to measure glomerular filtration rate directly in clinical practice, 24-hour endogenous creatinine clearance is used as a substitute for this index. 24-hour endogenous creatinine clearance formula = (urinary creatinine concentration (umol/L) * 24h urine volume (L) / (plasma creatinine concentration (umol/L). The normal value of endogenous creatinine clearance is between 80-120 ml/min in adults and between 40-65 ml/min in children. Endogenous creatinine clearance is used to classify the stage of chronic kidney disease. Usually endogenous creatinine clearance in chronic kidney disease is generally ≥90 ml/min in stage I, 60-89 ml/min in stage II, 30-59 ml/min in stage III, 15-29 ml/min in stage IV, and <15 ml/min in stage V. The lower the value of endogenous creatinine clearance, the worse the kidney function is represented.