Urea nitrogen is the end product of protein metabolism and is a nitrogenous compound in plasma other than protein, with normal reference values of 3.2-7.1 mmol/L for adults and 1.8-6.5 mmol/L for infants. urea is mainly synthesized in the liver and the kidneys are the main organ for urea excretion. After filtration from the glomerulus, about 30%-40% of urea is reabsorbed by the renal tubules . When the renal parenchyma is damaged, the glomerular filtration rate decreases, resulting in an increase in blood urea nitrogen concentration, so the clinical monitoring of urea nitrogen is mostly used to roughly observe the glomerular filtration function. Various renal parenchymal diseases, such as glomerulonephritis, interstitial nephritis, acute and chronic renal failure, and intrarenal occupying and destructive lesions, can increase the blood urea nitrogen.