Selection bias explains apparent differential mortality between dialysis modalities.

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.