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Nutritional Management in Peritoneal Dialysis
Protein-Energy Wasting
Patients with chronic kidney disease (CKD) are at risk for development of protein-energy malnutrition. The expert panels from the International Society of Renal Nutrition and Metabolism (ISRNM) first coined the term protein-energy wasting (PEW) to describe the malnutrition often seen among patients with CKD in 2007. PEW is specifically defined as the syndrome of depletion of systemic body protein and energy stores with specific diagnostic criteria. In this chapter, we will use PEW as an inclusive syndrome that also comprises protein and energy malnutrition and the complications associated with insufficient intake and increased catabolism seen in kidney disease patients.
Diagnosis and Assessment of Protein-Energy Wasting in Peritoneal Dialysis Patients
The ISRNM recommends using four separate criteria to diagnose PEW. These criteria include commonly utilized laboratory data, body mass measurement, muscle mass, and records of dietary intake to evaluate nutritional status. While not without some caveats, these criteria can be applied to peritoneal dialysis (PD) patients for the diagnosis of PEW and are summarized in Table 36.1 .
Table 36.1
Clinical Criteria for Protein-Energy Wasting Diagnosis in Dialysis Patients
Adapted from Fouque D, Kalantar-Zadeh K, Kopple J, et al. A proposed nomenclature and diagnostic criteria for protein-energy wasting in acute and chronic kidney disease. Kidney Int . 2008;73(4):391-398.
| Serum Chemistry |
|
| Body Mass |
|
| Muscle Mass |
|
| Dietary Intake |
|
At least three out of the four listed categories (and at least one test in each of the selected category) must be satisfied for the diagnosis of kidney disease-related PEW. Optimally, each criterion should be documented on at least three occasions, preferably 2–4 weeks apart.
- a.
Not valid if low concentrations are due to abnormally great urinary or gastrointestinal protein losses, liver disease, or cholesterol-lowering medicines.
- b.
A lower BMI might be desirable for certain Asian populations; weight must be edema-free mass, for example, postdialysis dry weight. See text for the discussion about the BMI of the healthy population.
- c.
Measurement must be performed by a trained anthropometrist.
- d.
Creatinine appearance is influenced by both muscle mass and meat intake.
- e.
Can be assessed by dietary diaries and interviews, or for protein intake by calculation of normalized protein equivalent of total nitrogen appearance (nPNA or nPCR) as determined by urea kinetic measurements.
BMI , Body mass index; CKD , chronic kidney disease; DEI , dietary energy intake; DPI , dietary protein intake; nPCR , normalized protein catabolic rate; nPNA , normalized protein of nitrogen appearance.
Serum Chemistry
Serum albumin is the most essential diagnostic criterion for PEW in CKD. Among PD patients, albumin per se has been shown to be negatively and linearly associated with mortality and poor outcomes. Importantly, low serum albumin levels in PD patients are not specific for PEW. Albumin level often fluctuates with volume status, and hypoalbuminemia is caused by protein loss in dialysis effluent and may occur without obvious signs of infection or inflammation. Nevertheless, hypoalbuminemia remains a powerful indicator of PEW and a significant prognosticator of long-term outcome. Because of its limitations, the National Kidney Foundation Kidney-Disease Outcomes and Quality Initiative (NKF-KDOQI) nutrition guideline also recommend the use of additional markers to evaluate PEW. Other useful laboratory measures to evaluate malnutrition and inflammation often include C-reactive protein, leptin, adiponectin, and prealbumin (also known as transthyretin). However, many of these laboratory markers are also prone to fluctuations due to inflammation or infection. Hence, to properly assess nutritional status, one should always evaluate the presence of any active inflammation because it will likely lead to anorexia and PEW. While low cholesterol level may be another surrogate marker for low oral intake, one needs to be cognizant of the abnormal lipid metabolism in kidney disease as well as the ubiquitous use and effect of lipid-lowering agents.
Body Mass
The diagnostic criteria for PEW includes an absolute body mass index (BMI) < 23 kg/m 2 or unintentional weight loss or low body fat. As in other CKD patients and those treated with hemodialysis (HD), the paradoxical relationship between high BMI and mortality, known as obesity paradox, is similarly observed among PD patients, albeit the findings are less consistent across studies. PD patients with low BMI have the highest risk for mortality, while higher BMI tends to be associated with a lower risk for death. It is important to know that in the general population, a BMI between 18.5 and 25 kg/m 2 may be considered the normal range, whereas ISRNM has suggested that a BMI < 23 kg/m 2 is in the lower range for dialysis patients.
Muscle Mass
Muscle mass is an important criterion for the diagnosis of PEW. Surrogates for muscle mass may be extrapolated from anthropometric measurements of mid-arm muscle circumference, waist circumference, or validated equations using serum creatinine. Serum creatinine is a useful surrogate of muscle mass in kidney failure patients when there is no residual kidney function. Studies have demonstrated that a very low creatinine level, < 4.0 mg/dL and 4.0–5.9 mg/dL, indicated a higher risk of death among PD patients with vintage > 1 year. Among PD patients, lean body mass has been found to correlate with creatinine kinetics, anthropometry, and bioelectrical impedance, but there are conflicting results on whether it may represent steady-state nutritional status or if it can prognosticate long-term outcomes.
Dietary Intake
PD patients with persistently low protein intake < 0.8 g/kg/day or daily energy intake < 25 kcal/kg/day are at particularly high risk for PEW. The estimation of caloric or protein intake is best estimated by a food intake diary or by normalized protein of nitrogen appearance (nPNA), also known as normalized protein catabolic rate (nPCR), as discussed later.
Other tools for PEW assessment recommended by the ISRNM are utilized commonly among clinicians and researchers during the care of PD patients. The following are examples of nutrition screening tools.
Normalized Protein of Nitrogen Appearance (Also Known as Normalized Protein Catabolic Rate)
Among patients in a steady state, urea nitrogen appearance is calculated to estimate daily protein intake, and nPNA can be used to estimate protein intake. In PD patients, protein of nitrogen appearance (PNA) can be calculated by estimating the generation of urea nitrogen in the dialysate and then normalized based on body weight. However, it is important to note the limitations of nPNA in PD. For example, among patients receiving intraperitoneal amino acid exchanges, the nPNA may be falsely elevated, giving an incorrect impression of a better nutritional state. Therefore, nPNA should be used in collaboration with other criteria to provide an accurate reflection of nutritional status.
Body Composition
Dual-energy x-ray absorptiometry or bioelectrical impedance analysis can be useful tools that inform clinicians of patients’ volume status and anthropometry. However, they are not often readily available for clinical use in most dialysis clinics. Importantly, intra-abdominal dwells of dialysate solutions will also change the accuracy of the bioimpedance measurement, and patients need to have the peritoneal dialysate drained prior to this measurement. Thus, bioelectrical impedance is not currently recommended for body composition assessment in PD.
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