Objective To compare estimates of protein balance using the urinary urea nitrogen method to predict total urinary nitrogen with isotopically derived estimates of metabolic protein balance as defined by the difference between rates of protein synthesis and breakdown.

Design Prospective, descriptive, repeated measures analysis. Urinary urea nitrogen collections were obtained for 8 to 24 hours before infusion of L-[1-¹³C] leucine during fed and fasted states.

Subjects/setting Eight acutely burned pediatric patients consecutively admitted to Shriners Burns Institute, Boston Unit, for medical and surgical care of their injuries.

Main outcome measures The difference between isotopically measured rates of protein synthesis and breakdown was used as an index of protein balance and compared with estimates of protein balance determined using the urinary urea nitrogen method.

Statistical analysis Least squares regression analysis was used to assess the value of urinary urea nitrogen as a predictor of metabolic protein balance. Limits of agreements were used to determine bias and precision between the two methods.

Results Urinary urea nitrogen was a significant predictor of metabolic protein balance (r²=.77; P<.001). The direction of protein balance was the same in 14 of 16 measurements; however, there was a significant lack of agreement between the two methods as demonstrated by large quantitative differences in protein balance.

Conclusion Although the urinary-urea-nitrogen-based estimates of protein balance correlate well with isotopically derived protein balance, they are not precise in determining protein balance in seriously burned children. J Am Diet Assoc. 1997; 97:489–495.