The kidneys play an important role in the metabolism of the body. The three kidney function tests performed in hospitals include blood creatinine, uric acid and urea nitrogen. Blood creatinine is the creatinine generated from creatine in muscles and enters the blood, which is filtered through the glomerulus and rarely absorbed by the renal tubules, and almost all of it can be excreted in the urine. Uric acid is produced by the metabolism of purines in the liver, while the genetic material DNA and RNA in cells in the human body can be broken down to produce purines, in addition to the intake of people through eating purine-rich foods. Most of the uric acid produced in the body is excreted in the urine by the kidneys, and a small amount is excreted in the feces and sweat. Most of the uric acid in the kidneys is reabsorbed, so the amount of uric acid in the urine is very small. Urea nitrogen is the end product of protein metabolism in the body. Amino acids are broken down into ammonia and carbon dioxide, both of which are synthesized into urea in the liver, and urea can contain up to half of the nitrogen, which is abundant and stable, and is often used as a test indicator. Urea can be reabsorbed in the renal tubules after filtration from the glomerulus, but in the urinary state, reabsorption is reduced and the body can clear urea. Abnormalities in the above three indicators may indicate abnormal kidney function, such as an increase in the ratio of blood urea nitrogen to blood creatinine in renal ischemia. In addition, the daily diet may also cause abnormal indicators, such as high protein diet and water deficiency may also cause an increase in the ratio; while a low protein diet will lower the ratio. Therefore, the three tests should be performed in the morning on an empty stomach. For further examination of kidney disease, detailed tests such as kidney ultrasound should be performed under the guidance of a doctor.