3 factors that can alter urinary urea nitrogen resulting in unreliable results?

• A. Creatinine clearance < 50 mL/min
• B. Low urine output
• C. Muscle atrophy
• D. All of the above

Answer: (D)

Urinary urea nitrogen is used to estimate nitrogen losses, but its accuracy hinges on complete, stable excretion and normal renal handling of urea. Low urine output makes 24-hour collection unreliable because the sample may miss a substantial portion of daily urea excretion, skewing the value. When creatinine clearance is markedly reduced, kidney function alters how urea is filtered and excreted, so the UUN may not reflect true nitrogen losses as expected. Muscle atrophy changes the body's protein pool and the rate of protein breakdown, often lowering urea production; this can make UUN appear lower than actual nitrogen losses for a given intake. Taken together, these factors can each distort UUN measurements, so all of the above can lead to unreliable results.