Selection bias explains apparent differential mortality between dialysis modalities

Abstract

The relative risk of death for patients treated with peritoneal dialysis compared with those treated with hemodialysis appears to change with duration of dialysis therapy. Patients who start dialysis urgently are at high risk for mortality and are treated almost exclusively with hemodialysis, introducing bias to such mortality comparisons. To better isolate the association between dialysis treatment modality and patient mortality, we examined the relative risk for mortality for peritoneal dialysis compared with hemodialysis among individuals who received ≥4 months of predialysis care and who started dialysis electively as outpatients. From a total of 32,285 individuals who received dialysis in Ontario, Canada during a nearly 8-year period, 6,573 patients met criteria for elective, outpatient initiation. We detected no difference in survival between peritoneal dialysis and hemodialysis after adjusting for relevant baseline characteristics. The relative risk of death did not change with duration of dialysis therapy in our primary analysis, but it did change with time when we defined our patient population using the more inclusive criteria typical of previous studies. These results suggest that peritoneal dialysis and hemodialysis associate with similar survival among incident dialysis patients who initiate dialysis electively, as outpatients, after at least 4 months of predialysis care. Selection bias, rather than an effect of the treatment itself, likely explains the previously described change in the relative risk of death over time between hemodialysis and peritoneal dialysis.

Figure 1.

There was no statistically significant difference in adjusted survival between individuals treated with PD and HD who had received at least 4 months of predialysis care and started dialysis electively, as outpatients. Adjusted survival curves were generated using the corrected group-prognosis method.

Figure 2.

Different definitions of chronic dialysis lead to different conclusions when mortality is compared among patients treated with PD compared with those treated with HD. This figure presents the relative hazard of death on PD versus HD over time, according to the presence or absence of diabetes mellitus, and the definition of chronic dialysis. In the Elective Outpatient Start cohort (top panel), there was no change in the relative hazard of death over time, and diabetes status did not influence the relationship between treatment modality and the risk of death. In both the All Outpatient Dialysis cohort (bottom left panel) and the 90-day cohort (bottom right panel), the risk of death on PD relative to HD rose over time and was higher in patients with diabetes compared with those without diabetes at any given time point.