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When one compares the clearances achieved by the "replacement" therapies (Figure 1) it is clear that these are not replacement therapies at all. Since all dialysis techniques are, at best, poor substitutes for the natural kidney, we need to strive to give as much dialysis to the individual patient as practical. In this context, interpretation of the CANUSA results is particularly enlightening.

In the CANUSA study, 680 new CAPD patients were enrolled in a prospective study in ten Canadian and four U.S. centers to determine the effect of clearance levels and nutrition on morbidity and mortality. The dialysis dose was at the discretion of the patient's individual investigator and was not increased as residual renal function (RRF) declined. In fact, mean peritoneal KT/V did not change over the two years of follow-up, and peritoneal creatinine clearance increased only slightly from 44.2 L/wk/1.73 m² at baseline to 47.3 L/wk/1.73 m² at 24 months (p < 0.005, baseline vs 24 months).

The CANUSA study data confirm that "more is better" in peritoneal dialysis. Two-year predicted patient survival data demonstrate that lower total solute clearances are associated with higher patient mortality (Table 1). Actuarial two-year survival was 78%. In a multivariate analysis, a 5 L/week/1.73 m² increase in total creatinine clearance was associated with a 7% decline in relative risk of death, and a 0.1-U increase in weekly KT/V was associated with a 6% decline in relative risk of death. It should be noted that changes in total creatinine clearance and KT/V in the CANUSA study are primarily due to decreases in residual renal function, rather than to alterations in peritoneal clearance. It is widely presumed, although unproven, that these findings can be extrapolated to support the existence of a similar association for alterations in peritoneal clearance.

Since the CANUSA data are usually presented in tabular form, the large difference in mortality between the predicted survival for a sustained creatinine clearance of 55 and 70 L/wk/1.73 m² has led some to consider 70 L/wk/1.73 m² the minimum creatinine target for CAPD. However, when these data are presented graphically with additional interpolated data points, the curve is smooth and does not plateau (Figure 2). While these data support the view that more total clearance is better, the suggested target levels can be questioned for several reasons:

1. Greater levels of total clearance are associated with better survival, thus, selecting one level over another is somewhat arbitrary
2. The analysis is complicated by the fact that RRF was responsible for almost all of the variation of total clearance.
3. The CANUSA study was unable to demonstrate a correlation between small molecule clearance and clinical outcomes independent of RRF because there was little variation in peritoneal clearance in the study population
4. The data are inferences from statistical modeling (6)
5. The data are based on uncontrolled, nonrand-omized cross-sectional correlations
6. Correlations based on clearances that have been measured every six months may significantly overstate the levels at which adverse events actually occur due to wide variances in the rate of RRF decline (7)

Peritoneal Dialysis International, Vol. 16, pp. 448­456 Printed in Canada. All rights reserved. Copyright © 1996 International Society for Peritoneal Dialysis 0896-8608/96 $3.00 + .00