The normal reference range of blood urea nitrogen varies slightly depending on the assay methodology, from 2.7-7.1 mmol/L for men and 2.6-7.5 mmol/L for women. urea nitrogen clinical significance includes two major aspects, one is increase and the other is decrease. First, the increase is seen in the increase of blood urea nitrogen in organic renal impairment, such as chronic renal failure due to various primary glomerulonephritis, pyelonephritis, interstitial nephritis, etc. Blood urea nitrogen is generally not used as an indicator of early renal impairment, but in chronic renal failure, especially in uremic patients, the increase of blood urea is usually consistent with the severity of the disease; second, it is also seen in pre-renal and post-renal factors. The former includes severe dehydration, massive ascites, cardiac circulatory failure, etc., and the latter, such as urinary tract obstruction caused by ureteral stones and other diseases; third, blood urea nitrogen can be used as an indicator to judge the adequacy of dialysis in renal failure. Lower urea nitrogen is seen in the following aspects: 1, reduced urea production causes lower urea, such as liver insufficiency, patients with cirrhosis, severe hepatitis, poisoning, etc. will cause lower urea; 2, low-protein diet, usually vegetarians and less vegetable protein such as soy products, peanuts, melon seeds, walnuts, etc., insufficient protein intake causes lower urea nitrogen; 3, higher protein demand in late pregnancy, infancy and childhood. Insufficient supplementation can cause lower urea nitrogen; 4, too much urea nitrogen excretion, resulting in lower urea nitrogen, such as polyuria, uremia and other diseases can cause lower urea nitrogen.