IV. Assessment of Nutritional Status Specifically as It Relates to Peritoneal Dialysis


Assessment of Nutritional Status (Opinion)

Nutritional status of adult PD patients should be assessed on an ongoing basis in association with Kt/Vurea and Ccr measurements using the Protein equivalent of Nitrogen Appearance (PNA) and Subjective Global Assessment (SGA). For pediatric PD patients, nutritional status should be assessed using the PNA and other standard nutritional assessments (see Guideline 14 of the Clinical Practice Guidelines for Peritoneal Dialysis Adequacy and the K/DOQI Clinical Practice Guidelines for Nutrition in Chronic Renal Failure).

Rationale A detailed rationale is presented in Appendix F. The following is a summary.

There is strong indirect evidence linking survival on dialysis with nutritional status both at initiation of dialysis (see Section I: Initiation of Dialysis) and during longitudinal follow-up. Better survival has been reported in PD patients with high normalized protein equivalent of nitrogen appearance (nPNA)50 (see Guideline 28: Measurement of Normalized PNA in PD Patients). Positive correlations between nPNA and clearance of urea or creatinine have been reported repeatedly in PD subjects.51-54 The correlation between nPNA and Kt/Vurea may indicate increased appetite and dietary protein intake as Kt/Vurea increases, but also may simply reflect the fact that nPNA and Kt/Vurea are mathematically linked.55 This mathematical linkage makes the correlation between nPNA and Kt/Vurea in cross-sectional studies of questionable clinical significance. However, nPNA tends to increase in the same subjects when Kt/Vurea and creatinine clearance (CCr) are increased by increasing the dose of PD,53 especially if the increase in the dose of PD was prescribed because of inadequate clearances.56 In the latter instance, the association between Kt/Vurea and nPNA is not the result of a mathematical coupling. In addition, there is strong evidence suggesting that quality and quantity of dialysis influences nutrition.57,58 While the precise relationship between kidney function (or dialysis therapy) and nutrition is not yet adequately understood, it is the Work Group’s opinion that adequate renal replacement therapy is necessary for normal appetite and metabolism. Thus, nutritional problems may reflect inadequate dialysis which, if corrected, may lead to subsequent improved outcomes.

Although nutritional status is influenced by many nondialysis-related factors, appetite suppression, nausea, and vomiting are major clinical features of uremia and inadequate dialysis. Therefore, nutritional status is also an important measure of PD adequacy. Of the available measures of nutrition, PNA is recommended because it provides an estimate of protein intake and protein losses. The SGA is recommended because it is a valid clinical assessment of nutritional status and is strongly associated with patient survival.

Protein equivalent of total nitrogen appearance (PNA) Nitrogen intake is almost entirely from protein. The final product of protein catabolism is urea. Therefore, if steady-state nitrogen balance conditions exist, one can work backward from urea excretion to determine what the protein intake was. Yet, other protein losses (urinary, peritoneal dialysate, diarrhea) also reflect the body’s turnover of protein. Studies in dialysis patients show that predictable mathematical relationships exist between urea excretion, protein catabolism, and dietary protein intake. If peritoneal protein losses are greater than 15 g/day, PNA should be calculated as protein catabolic rate + protein losses. If dialysate protein losses are less than 15 g/day, the formula:

PNA(g/d)=10.76*(0.69*UNA +1.46)

can be used to calculate PNA where UNA is total urea nitrogen appearance in grams per day.58a PNA is an indirect method for estimating dietary protein intake, a key measure of nutritional status in dialysis patients. There are alternative Bergstrom formulae to obtain the PNA surrogate for dietary protein intake:

PNA(g/24 hours)=15.1 + (6.95 × urea nitrogen appearance in g/24h) + dialystate and urine protein in g/24 hours59

In the absence of direct measurement of urinary and dialystate protein losses, this less accurate formula may be used:

PNA(g/24 hours)=20.1 + (7.50 × urea nitrogen appearance in g/24 h)

When protein losses are high, this second formula should not be used. Both formulae will require normalization to body mass in kg.

Subjective global assessment (SGA) The SGA is a simple assessment that uses the clinician’s experience to subjectively rate a patient’s nutritional status based on the medical history and physical exam. The SGA was modified for use in PD patients as described in Appendix F: Detailed Rationale for Guideline 12. This assessment is valuable because it does not focus on a single variable; rather, it forces the clinician to view the patient more broadly. SGA addresses four items (recent weight change, anorexia, subcutaneous tissue, and muscle mass) scored on a 7-point Likert scale (see Appendix B: Detailed Rationale for Guideline 2). It can be performed by physicians, nurses, or registered dietitians during routine clinic visits. Several studies have validated that the SGA accurately reflects nutritional status in dialysis patients and, in the CANUSA study, a higher SGA was associated with a lower risk of death.

The SGA is easy to perform and can be performed within minutes during a routine clinic visit. There are no data to dictate how often to perform the SGA, so the Work Group bases its opinion on the following: the SGA should be done often enough to detect changes and to intervene in a timely manner. It should be performed in association with measurement of Kt/Vurea and CCr, every 4 months after the initial 6 months (see Guideline 5: Frequency of Measurement of Kt/Vurea, Total CCr, PNA, and Total Creatinine Appearance).

The SGA has not been validated as a means of nutritional assessment in the pediatric PD population.


Determining Fat-Free, Edema-Free Body Mass (Opinion)

Total creatinine appearance should be used to determine fat-free, edema-free body mass.

Rationale Fat-free, edema-free body mass is probably a more accurate term for what had previously been called lean body mass. It is an important index of overall nutritional status. Total daily creatinine production, measured as the sum of creatinine excreted in dialysate and urine plus the estimated creatinine lost in the gut,28 can be used to calculate fat-free, edema-free body mass. Fat-free, edema-free body mass reflects somatic protein stores in the same way that serum albumin reflects visceral protein stores.60 In adults, fat-free, edema-free body mass in kilograms is computed by the equation60,61:

Fat-free, edema-free body mass = 0.029 × total creatinine production in mg/day + 7.38.

Norms vary by patient gender and size. A steady-state of creatinine excretion should exist for the equation result to be valid. Factors other than muscle mass can affect the fat-free, edema-free body mass calculation by creatinine kinetics. These factors include errors in the collection or measurement of creatinine in urine or dialysate and large variations in the dietary intake of creatine plus creatinine (meat). Fat-free, edema-free body mass estimates by creatinine kinetics may be a better index of nutritional status in PD patients, because they reflect dry fat-free, edema-free body mass and changes in muscle mass better than dual-energy x-ray absorptiometry or bioimpedance.62 The day-to-day variability of total creatinine excretion is only 2% to 4% over a short time interval,33,37 but is up to 15% over much longer intervals.27

Serum creatinine concentration is not, by itself, an index of adequacy of peritoneal dialysis because of the large variations in creatinine production between individuals. However, a change in serum creatinine concentration may indicate changes in creatinine and urea removal to a much larger extent than a change in serum urea concentration.63,64 A rising serum creatinine concentration is usually caused by a decrease in total creatinine clearance, often secondary to a loss of residual kidney function, and much less frequently by an increase in muscle mass. A decreasing serum creatinine concentration is caused more often by progressive loss in muscle mass and less often by an increase in total clearance. In other words, increases in peritoneal solute transport or recovery of kidney function do not occur very often.


Use of the Modified Borah Equation to Assess Nutritional Status of Pediatric PD Patients (Opinion)

Nutritional status of pediatric PD patients should be assessed at least every 6 months by standard clinical nutritional evaluations and by the modified Borah equation65:

PNA(g/d) = [6.49*UNA] + [0.294*V] + protein losses(g/day)

Rationale The equation described in Guideline 12, Assessment of Nutritional Status, from the glossary65 is a further modification of the original Borah equation.66 Since this modification has not been validated in children, the Work Group recommends using the modification above from Kopple et al65 and Keshaviah et al.67

Although not validated in children, this modified Borah equation contains a factor, V, that controls for patient size, and has been employed in pediatric studies. Furthermore, dialysate protein losses must be measured directly in the dialysis effluent and not estimated when using the modified Borah equation in children. The use of equations in which dialysate protein losses are estimated has been studied in very few pediatric PD patients.68

The dialysate protein measurement is the only additional laboratory determination from standard total solute removal measurements, as described in Guideline 4: Measures of PD Dose and Total Solute Clearance. Thus, this nutritional assessment could be easily merged to accompany total solute removal measurements.


The precise relationships among PNA, dialysis dose, and outcome are unclear. Although these relationships are currently being studied in hemodialysis patients, they should also be examined in PD patients. There are probably limits, for example, to the role of increasing delivered dialysis dose in order to improve appetite and nutritional parameters. Other questions of interest include: How does improved dialysis affect nutrition, and above what delivered dose does that effect dissipate, if at all? What interventions act synergistically with improved dialysis to improve nutritional parameters? What complications interfere with nutrition, in what manner, and can the interference be overridden by another type of intervention?

A standardized nutritional assessment tool for children analogous to the SGA should be developed.





© 2001 National Kidney Foundation, Inc

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