Urea Nitrogen, 24-Hour Urine

Urea Nitrogen, 24-Hour Urine (urine urea nitrogen, UUN) is used to assess protein metabolism and nitrogen balance, including during a nitrogen balance study. In critically ill patients receiving enteral or parenteral nutrition, it helps estimate ongoing protein catabolism and guides adjustment of protein intake during nutritional support. The test also serves as an adjunct when blood urea is increased, contributing to evaluation of renal excretory function and the kidney’s ability to eliminate urea.

Urea, an amide of carbonic acid, is the terminal product of amino nitrogen disposal via the hepatic urea (ornithine) cycle, which converts toxic ammonia into water‑soluble urea for renal elimination. Urea is freely filtered at the glomerulus; about one‑third of the filtered load undergoes passive tubular reabsorption, a process that increases when urine flow is reduced. Urea excretion parallels dietary protein intake. Under nitrogen equilibrium, daily excretion of approximately 500 mmol urea (≈14 g urea nitrogen) corresponds to consumption of roughly 100 g of protein. Urea constitutes about 90% of total urinary nitrogen. Increased urinary urea output indicates negative nitrogen balance, as seen with postoperative catabolism, thyrotoxicosis, or intestinal absorption of blood proteins after upper GI bleeding. Conversely, decreased urinary urea reflects positive nitrogen balance or impaired urea production or clearance. Low values occur with renal diseases, hepatic disorders that limit urea synthesis, and congenital deficiencies of urea cycle enzymes. Enhanced tubular reabsorption during hypovolemia or dehydration and reduced renal perfusion—such as with massive hemorrhage, shock, burns, or heart failure—also lower urinary urea; chronically diminished renal blood flow is frequent in older adults with atherosclerotic renal artery disease.