The normal value of adult urea nitrogen is 3.2-7.1mmol/L, infants and children 1.8-6.5mmol/L, urea nitrogen 2.7mmol/L may be mildly low, divided into physiological and pathological two cases, the former includes visible in vegetarians, generally not serious; the latter can be seen in patients with liver failure, more serious. Physiological conditions: urea nitrogen is the main end product of protein metabolism in the human body, usually vegetarian people, less protein intake, blood urea nitrogen may be low, this situation is not serious, it is recommended that appropriate protein supplementation. Pathologic conditions: including liver failure. Liver is the main organ of protein synthesis, liver failure patients with severe liver damage, coupled with insufficient protein intake, consumption is serious, urea nitrogen may also be low, this kind of situation is more serious. Urea nitrogen 2.7mmol / L is recommended to go to the hospital in a timely manner, under the guidance of the doctor to clarify the cause of the disease, if it is caused by pathological factors, need timely treatment.