nitrogen balance =
= (24 hour nitrogen intake in grams) - ((24 hour total urine nitrogen in grams) + 2)
24 hour nitrogen intake in grams =
= (0.16 * (dietary protein in grams)) + (0.168 * (crystalline amino acids for parenteral solutions in grams))
where:
• Nitrogen intake consists of 16% of dietary protein and/or 16.8% of crystalline amino acids used for parenteral nutrition.
• 2 indicates grams of nitrogen lost during usual skin sloughing and from fecal loss; Blackburn uses a factor of 4 instead
• The equation assumes no diarrhea and no excessive loss via other routes; in the event of these disorders, direct measurement of nitrogen loss should be performed.
Testing:
(1) 24 hour urine specimens are more accurate than extrapolations from shorter urine collections
(2) Total urine nitrogen should be measured directly when possible. While the total nitrogen can be approximated from the urine urea nitrogen (about 80% of total nitrogen is urea nitrogen), this can be misleading in critically ill patients.
Interpretation:
• A positive nitrogen balance of 3-to-5 grams per day is sufficient for anabolism and wound healing.
• Negative nitrogen balance often occurs for several days following acute injury.
