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Article Review/Hyperlink
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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.
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