Фракционная экскреция мочевины (FEUrea) (венозная кровь, суточная моча)

Urine urea nitrogen (UUN), measured on a 24-hour urine collection, is used to assess nitrogen balance and the dynamics of protein catabolism and synthesis. The UUN test helps tailor protein prescriptions for critically ill patients receiving enteral or parenteral nutrition and assists in evaluating renal excretory function when serum urea is elevated. Synonyms such as urea in 24-hour urine and urine urea nitrogen are commonly used in clinical practice.

Urea is the amide of carbonic acid and the principal end product of protein metabolism. It is synthesized in hepatocytes via the urea cycle, a sequence of enzyme-mediated reactions that converts toxic ammonia to water-soluble, relatively low-toxicity urea for renal elimination. After glomerular filtration, approximately 35% of filtered urea undergoes passive tubular reabsorption, a process that increases as urine flow diminishes. Urinary urea excretion tracks dietary protein intake. In adults at nitrogen equilibrium, excretion of 500 mmol of urea (equivalent to 14 g of urine urea nitrogen) per day corresponds to an intake of about 100 g of protein. Urea accounts for roughly 90% of the nitrogenous metabolites eliminated in urine. Interpretation reflects whole-body protein balance and renal handling. Increased urinary urea excretion indicates a negative nitrogen balance and is seen postoperatively, with hyperthyroidism, and with intestinal absorption of blood constituents after upper gastrointestinal bleeding. Reduced urinary urea excretion suggests a positive nitrogen balance and occurs with kidney disease and with some liver disorders due to impaired urea synthesis; it is also observed in congenital deficiencies or absence of urea-cycle enzymes. A lower urine urea concentration can result from augmented passive reabsorption in states of dehydration, hypovolemia, and reduced renal perfusion, including massive hemorrhage, shock, dehydration, burns, and heart failure. Diminished renal blood flow is likewise common in older adults with atherosclerosis of the renal arteries and impaired kidney function. Clinically, the urine urea nitrogen (UUN) measurement is used to gauge the relative rates of protein breakdown and synthesis to inform protein requirements in patients receiving enteral or parenteral nutrition, especially in intensive care. When serum urea is elevated, concurrent assessment of urinary urea helps characterize renal excretory performance; high blood urea accompanied by low urinary excretion commonly indicates renal failure.