BackgroundPrevious studies have found inconsistent associations between pre-transplant dialysis modality and subsequent post-transplant survival. We aimed to examine this relationship using the instrumental variable method and to compare the results with standard Cox regression.MethodsWe included 29 088 patients (age >20 years) from 16 European national or regional renal registries who received a first kidney transplant between 1 January 1999 and 31 December 2008 and were on dialysis before transplantation for a period between 90 days and 10 years. Standard multivariable Cox regression examined the association of individually assigned pre-transplant dialysis modality with post-transplant patient and graft survival. To decrease confounding-by- indication through unmeasured factors, we applied the instrumental variable method that used the case-mix adjusted centre percentage of peritoneal dialysis (PD) as predictor variable.ResultsStandard analyses adjusted for age, sex, primary renal disease, donor type, duration of dialysis, year of transplantation and country suggested that PD before transplantation was associated with better patient [hazard ratio, HR (95% CI) = 0.83 (0.76-0.91)] and graft survival (HR (95% CI) 0.90 (0.84-0.96)) when compared with haemodialysis (HD). In contrast, the instrumental variable analysis showed that a 10% increase in the case-mix adjusted centre percentage of patients on PD was neither associated with post-transplant patient survival [HR (95% CI = 1.00 (0.97-1.04)] nor with graft survival [HR (95% CI) = 1.01 (0.98-1.04)].ConclusionsThe instrumental variable method failed to confirm the associations found in standard Cox regression between pre-transplant dialysis modality and patient and graft survival after transplantation. The lack of association in instrumental variable analysis may be due to better control of residual confounding.