Clinical Information

Urea is the major product of protein nitrogen metabolism. It is synthesized by the urea cycle in the liver from ammonia which is produced by amino acid deamination. The majority of serum urea is excreted by the kidneys, thus, increased urea in the blood is a major indicator of renal disease.

Elevations in blood urea nitrogen concentration (azotemia) can occur due to prerenal, renal, and post renal causes. Prerenal azotemia is seen in inadequate renal perfusion (heart failure), shock, and diminished blood volume; renal azotemia occurs with chronic nephritis, nephrosclerosis, tubular necrosis and glomerular nephritis; and post renal azotemia is caused by urinary tract obstruction and malignancy. When used in conjunction with serum creatinine determinations, urea can help differentiate between these three types of azotemia as creatinine increases greatly with renal azotemia, only slightly with post renal azotemia and not at all in prerenal azotemia. Creatinine is a more specific indicator of renal disease as urea can also be transiently increased during periods of high protein intake, during treatment with cortisol, or with liver diseases.

Blood urea nitrogen (BUN) is a historical term from when urea assays used whole blood. The term is still commonly used in the USA to order urea assays on serum, plasma, and urine.