Protein nitrogen appearance in CAPD patients: what is the best formula?

BACKGROUND.: The protein equivalent of nitrogen appearance is an indirectindex commonly used to assess dietary protein intake in patientson CAPD. Moreover it has been suggested that the ratio betweennitrogen appearance and dietary nitrogen intake (fractionalurea synthesis) can predict nitrogen balance in uraemic patients.Several formulae to directly calculate the protein equivalentof nitrogen appearance have been published. It has not beenestablished, however, what formulae give the most appropriateestimate of protein intake and nitrogen appearance. STUDY DESIGN.: Nitrogen balance studies were carried out in seven stable patientson CAPD. All of the patients were receiving a diet whose proteincontent (1.2 g/kg/body wt/day) and calorie content (35 kcal/kg/bodywt/day) were rigorously controlled. Six formulae for calculatingprotein equivalent of nitrogen appearance and nitrogen appearancewere tested and the agreement of the estimating formulae wasevaluated by means of the Bland and Altman method. RESULTS.: Net nitrogen balance was 1.68±0.9 g/N day, protein intake(g/day) 81±19, protein intake (g/kg) 1.05±0.17.Differences in protein equivalent of nitrogen appearance ofup to about 20% were found. The smallest differences betweenprotein equivalent of nitrogen appearance and protein intakewere obtained by the formulae of Bergstrom (1±7 g, limitsof agreement –12 and +15 g) and Blumenkrantz (–2±5g, limits of agreement –11 and +7 g). The formula of Bergstrommost closely estimated nitrogen appearance (–0.35±0.89g). Using such formula, the fractional urea synthesis was 54±12%,giving evidence of positive nitrogen balances. CONCLUSION.: For the routine monitoring of protein equivalent of nitrogenappearance in CAPD patients, we recommend Bergstrom's formulawith the determination of dialysate protein losses.