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Using Normalized Protein Nitrogen Appearance (nPNA) in Assessing Nutrition
Kara Abbas
| The Issues in Renal Nutrition
in Nephrology Nursing department is designed to focus on nutritional
issues for nephrology patients. Address correspondence to: Deborah Brommage,
Department Editor, Nephrology Nursing Journal; East Holly Avenue/Box
56; Pitman NJ 08071-0056; (856) 256-2320. The opinions and assertions
contained herein are the private views of the contributors and do not
necessarily reflect the views of the American Nephrology Nurses'
Association. |
Protein-energy
malnutrition is very common among people with CKD Stage 5 and is
associated with increased infection rate, and even increased risk of
cardiovascular disease (Kalantar-Zadeh, Supasyndh, Lehn, McAllister,
& Kopple, 2003). There is no single measurement that can provide a
complete overview of nutritional status. Studies are still being
conducted to determine the most effective combination of measurements
to determine nutritional status. The K/DOQI Clinical Practice
Guidelines for Nutrition in Chronic Renal Failure (National Kidney
Foundation [NKF], 2000) suggest performing the following measurements
routinely in all people with Stage 5 CKD and on renal replacement
therapy: pre-dialysis serum albumin, % of usual post-dialysis
hemodialysis (HD) or post-drain peritoneal dialysis (PD) body weight, %
standard body weight (NHANES II), Subjective global assessment, dietary
interview and/or diaries, and normalized protein nitrogen appearance
(nPNA) or normalized protein catabolic rate (nPCR).
What is nPNA?
Several
studies have found that PNA is highly correlated with both mortality
and morbidity (Gotch & Sargent 1995; Kalantar-Zadeh et al., 2003;
Schoenfield , Henry, Laird, & Roxe, 1983). The K/DOQI guidelines
state that “PNA or PCR is a valid and clinically useful measure of net
protein degradation and protein intake in maintenance dialysis patients
(evidence)” (NKF, 2000, p. S29). PNA expresses total nitrogen
appearance in terms of protein in grams per day and can be normalized
to grams per kilogram per day (nPNA). These calculations are based on
the fact that the nitrogen content of mixed proteins is approximately
16% and most of the nitrogen lost from the body are either metabolites
of protein (urea) or are actual proteins and peptides (in urine and
peritoneal dialysate). Since PCR is part of formal urea kinetic
modeling, it is routinely checked monthly in people on hemodialysis and
quarterly in people on peritoneal dialysis. PNA and PCR – Where Do the Numbers Come From?
According
to McCann (2002), formal Urea Kinetic Modeling (UKM) is the gold
standard for assessing PNA because it allows for a direct calculation
of PCR. Formal UKM requires actual dialysis treatment parameters, such
as blood flow rate, dialysate flow rate, dialyzer, time, pre and post
weights, pre and post BUN levels, residual renal urea clearance (KrU),
and height for hemodialysis. The Natural Log method requires time on
dialysis, ultrafiltration rate, and height. This method does not allow
for direct calculation; however it is the only other alternative in
calculating PCR if formal UKM is not used. Urea Reduction Ratio (URR),
and Kt/V derived from URR do not allow for PCR calculations. See Table
1 for differences between PCR and PNA. PNA is a more accurate
terminology if there is a urine output or protein loss.
Using nPNA
nPNA
can be a good marker of protein intake if used in conjunction with
other biochemical markers, and stated intake/diaries. It is important
for the nurses and technicians to verify that the correct treatment
information is documented when UKM bloodwork is drawn since the result
is dependant on the treatment parameters. Nurses and dietitians can
then use nPNA to assess response to nutritional intervention since it
has a quick response to any changes in protein intake. nPNA can also be
used to assess for protein losses in urine and PD fluids, metabolic
status, and adequacy of dialysis. See Table 2 for result significance
and suggested interventions.
Considerations And Limitations
There
are some important considerations/limitations when using PNA. PNA
approximates protein intake only when at steady-state. If a person is
catabolic, PNA will exceed protein intake. Conversely, when a person is
anabolic (growth in children, recovering from illness, or pregnancy)
the dietary protein is being utilized and the PNA will underestimate
actual protein intake. PNA may also fluctuate from day to day as this
measurement will change rapidly depending on actual protein intake. It
is also strongly recommended to normalize PNA to body weight,
especially in those people less than 90% or greater than 115% of
Standard Body Weight (SBW) since it can be misleading in people who are
obese, malnourished, and edematous (McCann, 2002)
References
Gotch,
F.A., & Sargent, J.A. (1995). A mechanistic analysis of the
National Cooperative Dialysis Study (NCDS). Kidney International, 28,
526-534.
Kalantar-Zadeh, K., Supasyndh, O., Lehn,
R.S., McAllister, C.J., & Kopple, J.D. (2003). Normalized Protein
Nitrogen Appearance (PNA) is correlated with hospitalizations and
mortality in hemodialysis patients with Kt/V greater than 1.2. Journal
of Renal Nutrition, 13, 15-25.
McCann, L. (Ed.) (2002). Pocket guide to
nutrition assessment of the patient with chronic kidney disease (3rd
ed.). New York, NY: National Kidney Foundation.
National Kidney Foundation. (2000). K/DOQI
clinical practice guidelines for nutrition in chronic renal failure.
American Journal of Kidney Diseases, 35(Suppl 2), S1-S140.
Schoenfield, P.Y., Henry, R.R., Laird,
N.M., & Roxe, D.M. (1983). Assessment of nutritional status of the
National Cooperative Dialysis Study population. Kidney International,
Suppl 13, S80-S88.
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