OBJECTIVE: To test whether better nutrition is associated more with adequate urea clearance than with inadequate urea clearance in obese patients on continuous peritoneal dialysis (CPD). DESIGN: Retrospective analysis of clearance and nutrition indices in obese CPD patients. Only obese patients were analyzed. Obesity was defined as a ratio of actual weight to desired weight (W/DW) > or = 1.2. The dose of dialysis was considered adequate at weekly Kt/V urea > or = 2.0. Small solute clearances and nutrition indices were compared between patients with weekly Kt/V urea < 2.0 and patients with weekly Kt/V urea > or = 2.0 at the first clearance study. SETTING: Four university-affiliated and two private dialysis units in Canada and the United States. PATIENTS: A total of 270 CPD patients with W/DW > or = 1.2 at the first clearance study. RESULTS: Among the 270 obese CPD patients, 157 (58.1%) were underdialyzed (weekly Kt/V urea 1.66 +/- 0.22) and 113 (41.9%) had adequate dialysis (weekly Kt/V urea 2.51 +/- 0.47) at the first clearance study. Creatinine clearance values also differed between the underdialyzed and adequately dialyzed obese groups (55.6 +/- 15.2 vs 87.6 +/- 29.8 L/1.73 m2 weekly, respectively, p < 0.001). The underdialyzed group contained fewer women (39.5% vs 60.2%, p < 0.001) and more patients with anuria (35.0% vs 8.8%, p < 0.001), and had higher serum urea (20.7 +/- 6.9 vs 18.2 +/- 5.3 mmol/L, p = 0.001) and serum creatinine (974 +/- 283 vs 734 +/- 275 micromol/L, p < 0.001), marginally lower serum albumin (35.8 +/- 5.2 vs 37.2 +/- 6.4 g/L, p = 0.082), lower urea nitrogen excretion (5778 +/- 2290 vs 7085 +/- 2238 mg/24 hr, p < 0.001) and indices derived from urea nitrogen excretion (protein nitrogen appearance and normalized protein nitrogen appearance), and lower creatinine excretion (1034 +/- 349 vs 1217 +/- 432 mg/24 hr, p < 0.001) and indices derived from creatinine excretion (lean body mass normalized to actual or desired weight) than the adequately dialyzed group. CONCLUSION: Nutrition indices derived from urea nitrogen and creatinine excretion are worse in underdialyzed than in adequately dialyzed obese CPD patients. This finding may have clinical importance, despite the mathematical coupling between small solute clearances and excretion rates in cross-sectional studies, because of evidence from other studies that small solute excretion rate in cross-sectional studies is a robust Independent predictor of outcome in CPD.

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