HDCN Article Review/Hyperlink

Mandolfo S, Zucchi A, D'Oro LC, Corradi B, Imbasciati E

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

Nephrol Dial Transplant (Aug) 11:1592-1596 1996

There are now multiple formulae which can be used to calculate protein catabolic rate [PCR] or protein nitrogen appearance [PNA], as it is sometimes called in peritoneal dialysis [PD] patients. These formulae can give quite different values for PNA. One of the problems may be that most of the formulae are indirectly derived from hemodialysis and do not take into account the higher non-urea, non-protein nitrogen excretion in PD.

In this paper the authors study 7 middle-aged male CAPD patients in a hospital ward, where they are prescribed an optimal protein and caloric intake to ensure neutral or slightly positive nitrogen balance. Protein intake is carefully measured as are urea and protein losses in urine and dialysate. The measured protein intake and the calculated urea nitrogen appearance [UNA] are compared with the values obtained using the various formulae. The authors find that the formulae give widely different results but that the most accurate for calculation of the PNA are the Bergstrom and Blumenkrantz formulae. In the case of UNA, the most accurate is the Bergstrom formula. In this study the authors are referring to the Bergstrom formula which includes dialysate protein losses as a separate measured item, as distinct from an estimate.

This is an important paper and validates the Bergstrom formula as the most suitable for use in PD patients. It appears that other formulae tend to underestimate protein intake in PD patients.

Comment: A number of points are worth making, however. Firstly, the validation is based on a relatively small number of middle-aged males taking optimal diets. Would similar results be found in an older, predominantly female population? Also the authors do not directly measure non-urinary nitrogen losses but rather presume them to be 31 mg/kg/day. However, Bergstrom has shown in his 1993 KI paper that the figure is nearer 51 mg/kg/day for PD patients, likely related to greater non-urea nitrogen losses in dialysate. The authors refer to the use of fractional urea synthesis as a predictor of nitrogen balance. They suggest that values less than 80% suggest positive balance while values greater than 85% suggest negative balance. This prediction assumes that non-urea nitrogen losses are constant between the different dialytic modalities. In fact, as already stated, the non-urea nitrogen losses may be substantially greater on PD as compared to hemodialysis, and so a lower fractional urea synthesis on PD may reflect this rather than more positive nitrogen balance.

Despite these criticisms this is an excellent paper and I would suggest that the Bergstrom formula, with measurement of actual dialysate protein losses, should become a standard for the PD community. (Peter G. Blake, M.D., Victoria Hospital, London, Ontario)

The full text of this abstract is available from Oxford Press at this site.