Urea Nitrogen Measurement of urea concentration in serum is included in chemistry profiles mainly to screen for decreased glomerular filtration rate (GFR). Urea concentration is measured as urea nitrogen, and the test is usually called blood urea nitrogen (BUN) or serum urea nitrogen (SUN). Concentrations of UN are expressed in mg/dL. Urea is synthesized by hepatocytes from ammonia generated by catabolism of amino acids derived either from digestion of proteins in the intestines or from endogenous tissue proteins. Urea is excreted by the kidneys, intestine (high in horses), saliva and sweat. In ruminants, urea is excreted into the gastrointestinal system where it is converted to amino acids and ammonia which are then used for protein production (remember urea is added as a supplement to many bovine diets). Concentrations of UN are dependent upon: Hepatic urea production: The rate of urea production is dependent on hepatic function and digestion and catabolism of protein, i.e. urea formation is decreased in liver disease (e.g. portosystemic shunts) and increased with protein catabolism or increased protein digestion in the intestine. Renal tubular flow rate: Urea is freely filtered through the glomerulus and passively diffuses out of the tubules at a rate dependent on flow rate through the tubules; the remainder of the filtered urea is excreted in urine. At high flow rates, approximately 40% of filtered urea is reabsorbed. At low flow rates, as happens in hypovolemic individuals, approximately 60% of filtered urea is reabsorbed and added back to the blood urea concentration. This explains the high UN levels seen with decreased GFR of any cause. Causes of increased UN Increased protein catabolism: Fever, burns, corticosteroid administration, starvation, exercise. Increased protein digestion: Hemorrhage into the gastrointestinal system, high protein diets. Decreased GFR: Due to pre-renal, renal or post-renal causes. Note that UN will be increased with a normal creatinine in the following situations: Increased production of urea, e.g. protein catabolism. Early prerenal azotemia (most causes of a high UN in horses and ruminants are due to prerenal causes). Artefactual depression of creatinine, e.g. severe icterus. Causes of decreased UN Decreased protein intake or protein anabolism: Dietary restriction of protein, young animals (high anabolic rate). Increased excretion: Any cause of polyuria, e.g. hyperadrenocorticism, diabetes mellitus. Decreased production: Liver disease. Copyright © Cornell University