Urea Nitrogen

Urea nitrogen, 24-hour urine is used to assess nitrogen balance and the degree of protein breakdown, allowing calculation of daily protein and nitrogen needs in critically ill individuals on enteral or parenteral nutrition. The test, often termed urine urea nitrogen (UUN), is incorporated into nutrition support plans to adjust protein provision. When serum urea is elevated, urinary urea nitrogen helps evaluate renal excretion and distinguishes reduced kidney clearance from increased urea generation due to catabolic states or high protein intake. Interpretation is made in the context of clinical status and, when indicated, alongside serum urea nitrogen (blood urea nitrogen, BUN) and creatinine.

Urea is generated in the liver via the urea cycle from ammonia derived from amino acid catabolism and is eliminated by the kidneys. Urea is freely filtered at the glomerulus; about one-third is passively reabsorbed, a process that is accentuated when urine flow is low. Urinary urea output is proportional to dietary protein intake and typically accounts for about 90% of total urinary nitrogen. Higher urinary urea excretion indicates negative nitrogen balance and is seen with postoperative catabolism, hyperthyroidism, or intestinal absorption of blood proteins after upper gastrointestinal bleeding. Reduced excretion reflects positive nitrogen balance and occurs with renal diseases, impaired hepatic urea synthesis—including urea cycle enzyme deficiencies—and under the influence of certain hormones such as growth hormone, testosterone, and insulin. Lower urinary urea concentrations may also result from increased tubular reabsorption in settings of reduced renal perfusion or dehydration, including hemorrhage, shock, dehydration, burns, or heart failure, and in older adults with renal arterial atherosclerosis. Results are interpreted together with the clinical picture and, when appropriate, with serum urea nitrogen and creatinine.