Plasma creatinine is endogenous and its plasma concentration is relatively stable and does not require intravenous infusion to maintain a constant blood concentration as inulin does. Plasma creatinine is essentially filtered from the glomerulus, with only a small portion secreted by the tubules and not reabsorbed by the tubules. Endogenous creatinine clearance (ccr) can be used as a substitute for inulin clearance (cin) as a clinical measurement of gfr. Measurement method: ① Diet: Traditional view is that meat should be prohibited for 3 days before the experiment. According to the observation in recent years, eating 250g of lean meat daily has no statistically significant effect on scr and ccr. ② Avoid strenuous exercise before and during the test, so as not to increase blood and urine creatinine concentration. ③ Retention of urine: in order to reduce the error caused by the bladder can not be completely emptied, generally leave 24 hours of urine (the morning of the day to the next morning), urine with preservatives. The patient must be taught to retain urine accurately and asked to repeat it. Avoid passing urine with stool. ④ At the end of urine retention on the following morning, 2 ml of fasting blood should be collected and sent for testing to measure serum creatinine (scr). It is best to take blood for testing on the same day that urine retention begins, and take the average of the two times of scr. Measure the 24-hour urine output and send a sample for urine creatinine (ucr) testing. According to the scr, ucr and urine volume into the formula that is ccr. Calculate the results and then according to the standard body surface area correction. Corrected value = ccr × standard body surface area / actual body surface area. The standard body surface area is 1.73m2 for Europeans and Americans, and 1.48m2 for Japanese. Normal value: 108±15.1ml/min・1.73m2, which gradually decreases after 40 years of age.