Which of the following compromises the reliability of urinary urea nitrogen to calculate nitrogen balance?

• A. Protein intake < 0.5 g/kg/day
• B. Creatinine clearance < 50 mL/min
• C. Diuresis > 2.5 L
• D. Fecal nitrogen > 1 g

Answer: (B)

The main idea is that nitrogen balance calculations using urinary urea nitrogen assume that urinary nitrogen excretion faithfully reflects total nitrogen losses. This relies on normal kidney function and a complete 24-hour urine collection. When kidney function is significantly impaired, as indicated by a creatinine clearance well below normal, urea handling by the kidneys becomes unreliable. Urea production may stay the same, but its excretion no longer tracks predictably with intake, so urinary urea nitrogen no longer accurately represents total urinary nitrogen losses. That makes the nitrogen balance estimate unreliable.

Other factors can complicate the measurement, such as very high urine output raising the risk of incomplete collection or fecal nitrogen losses varying from the assumed non-urinary loss, but the strongest, most direct threat to the reliability of using UUN for nitrogen balance is markedly reduced renal function.