A surgical ICU patient on enteral feed receives 100 g of protein per day. His 24-hour urinary urea nitrogen excretion is measured at 8 g/day. Using the standard nitrogen balance equation that accounts for approximately 4 g of unmeasured nitrogen losses, what is his daily nitrogen balance and what does it indicate?

• A Minus 4 g/day, indicating catabolism
• B Zero, indicating equilibrium
• C Plus 8 g/day, indicating net anabolism
• D Plus 4 g/day, indicating net anabolism ✓

Explanation

Protein nitrogen content is approximated at 16 percent, so 100 g of protein provides 16 g of nitrogen. Nitrogen balance equals nitrogen intake minus (urinary urea nitrogen plus 4 g for faecal, skin and other unmeasured losses): 16 minus (8 plus 4) equals plus 4 g/day, indicating net anabolism. Option C forgets the 4 g correction factor for unmeasured losses, and option A reverses the sign. A positive balance of 2 to 4 g/day is the usual target in recovering surgical patients.

Reference: Sabiston Textbook of Surgery, 21st ed.

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