Protein is first broken down by digestion into amino acids. The normal liver converts this ammonia to urea which is then easily excreted via the kidneys. As can be seen above, because urea is formed in the liver it is not only used to monitor kidney function but can indicate liver pathology as well when normal kidney function is present. This would show as a very low serum urea level. Further liver function tests would be required to categorise the liver status.
Most renal diseases result in poor excretion of urea and therefore serum levels begin to climb.
These are normally accompanied by an increase of another waste product called creatinine which will be discussed next. In renal failure very high levels of urea can be seen if the patient is not on dialysis. Urea levels are also marginally affected by the degree of hydration and mild increases are commonly seen in dehydrated patients. These levels return to normal when the dehydration is corrected. The amount of creatinine produced daily is directly related to the muscle mass of the individual.