This test measures the amount of urea nitrogen in the blood. Urea is produced in the liver when protein is broken into its component parts (amino acids) and metabolized. This process produces ammonia, which is then converted into the less toxic waste product urea. Urea is released by the liver into the bloodstream and is carried to the kidneys, where it is filtered out of the blood and excreted in the urine. Since this is an ongoing process, there is usually a small but stable amount of urea nitrogen in the blood. Sample: Blood (serum); urine Reference: Between 13 mg/dl and 43 mg/dl Frequency: Daily