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Assessing nutritional status (protein malnutrition)
Evaluating protein catabolism
Determining nitrogen balance, when used in conjunction with 24-hour fecal nitrogen measurement
Nitrogen is a key component of proteins. During protein catabolism (proteolysis), nitrogen is excreted in the urine and feces. Protein catabolism may be increased after stress (eg, physical trauma, surgery, infections, burns).
"Nitrogen balance" is the difference between the amount of nitrogen ingested and the amount excreted. A patient who is in negative nitrogen balance is using muscle protein to meet the metabolic requirements of the body and is, therefore, in a catabolic state.
<16 years: not established
> or =16 years: 4-20 g/24 hours
Twenty-four hour urinary nitrogen excretion levels within the normal range are indicative of adequate nutrition. Slightly abnormal excretion rates may be a result of moderate stress or complications such as infection or trauma. Significantly abnormal excretion rates may be associated with severe stress due to multiple trauma, head injury, sepsis, or extensive burns. The goal with therapy for a depleted person is a positive nitrogen balance of 4 to 6 g nitrogen/24 hours.
Measurement of both urine and fecal nitrogen is necessary for the accurate determination of nitrogen balance.
During nitrogen balance studies, nitrogen lost from exuding wounds, such as burns, and from copious sputum must be included in the patient's evaluation.
1. Phinney SD: The assessment of protein nutrition in the hospitalized patient. Clin Lab Med 1981;1:767-774
2. Veldee MS: Nutritional assessment, therapy, and monitoring. In Tietz Textbook of Clinical Chemistry. Third edition. Edited by CA Burtis, ER Ashwood. Philadelphia, WB Saunders Company, 1999, pp 1385-1386
3. Konstantinides FN, Kostantinides NN, Li JC, et al: Urinary urea nitrogen: too insensitive for calculating nitrogen balance studies in surgical clinical nutrition. J Parenter Enteral Nutr 1991;15:189-193