Blood urea nitrogen
The blood urea nitrogen test (BUN) is used to measure the nitrogen fraction of urea, the chief end product of protein metabolism. Formed in the liver from ammonia and excreted by the kidneys, urea constitutes 40% to 50% of the blood's nonprotein nitrogen. The BUN level reflects protein intake and renal excretory capacity, but is a less reliable indicator of uremia than the serum creatinine level. BUN values normally range from 8 to 20 mg/dl and show slightly higher in the elderly patient. Elevated BUN levels occur in renal disease, reduced renal blood flow, urinary tract obstruction, and increased protein catabolism (such as burns). Low BUN levels occur in severe liver damage, malnutrition, and overhydration.