Urea Nitrogen, 24-Hour Urine

Urea Nitrogen, 24-Hour Urine (also called urine urea nitrogen or UUN) is used to assess protein metabolism and nitrogen balance, including appraisal of catabolic versus anabolic states during enteral or parenteral nutrition. It informs nutrition dosing in critical illness by estimating nitrogen losses and tracking response to dietary adjustments. The test also aids interpretation of azotemia by helping differentiate reduced urinary elimination from excess urea generation, supporting clinical evaluation of renal excretory function in the setting of kidney disease.

The liver converts ammonia to urea via the urea cycle, creating a water‑soluble compound that is cleared by the kidneys. Urea is freely filtered at the glomerulus, and about one‑third is passively reabsorbed; reabsorption increases when urine flow is low. Daily urinary urea output correlates with protein intake. At nitrogen equilibrium, excreting about 500 mmol of urea (≈14 g urea nitrogen) corresponds to ≈100 g of dietary protein. Approximately 90% of nitrogenous waste excreted in urine is urea. Elevated urinary urea excretion indicates negative nitrogen balance and may be observed after surgery, in hyperthyroidism, or when blood proteins are absorbed from the gastrointestinal tract following upper gastrointestinal bleeding. Reduced urinary urea excretion suggests positive nitrogen balance or impaired urea production/excretion and occurs with kidney disease, liver disorders that limit urea synthesis, and with administration of hormones such as growth hormone, testosterone, and insulin. Lower excretion can also reflect augmented tubular urea reabsorption during states of low urine flow due to dehydration, hypovolemia, or decreased renal perfusion (eg, hemorrhage, shock, burns, heart failure), and in older adults with renal arterial atherosclerosis.