title: Confounding effect of comorbidity in survival studies in patients on renal replacement therapy.
abstract: BACKGROUND
  After taking other confounding factors into account, the impact of comorbidity on mortality was investigated when comparing mortality between five European countries, dialysis modalities and renal disease groups.
  
  
  METHODS
  The study included 15 571 incident patients on renal replacement therapy (RRT) from five national or regional registries participating in the European Renal Association-European Dialysis and Transplant Association Registry that collect comorbidity data. The presence of diabetes mellitus, ischaemic heart disease, peripheral vascular disease, cerebrovascular disease and malignancy was recorded at the start of RRT.
  
  
  RESULTS
  The comorbidities were each independently associated with mortality, with hazard ratios (HRs) ranging from 1.40 (95% CI: 1.30-1.51) for peripheral vascular disease to 1.65 (95% CI: 1.48-1.83) for diabetes. Age, gender, primary renal disease, modality and country together explained 14.4% of the variance in mortality; the comorbidities explained an additional 1.9%. In the comparison of renal vascular disease with glomerulonephritis, the crude HR of 2.40 (95% CI: 2.12-2.72) changed to 1.24 (95% CI: 1.09-1.41) after adjustment for age, gender, primary renal disease, treatment modality and country and to 1.06 (95% CI: 0.93-1.22) after further adjustment for the comorbidities. For the comparison between countries and other patient groups, the change in the survival estimate after adjustment for comorbidity was less.
  
  
  CONCLUSIONS
  Comorbidity is an important predictor for mortality. However, after adjustment for age, gender, primary renal disease, treatment modality and country, when comparing outcomes between patient groups the influence of comorbidity may be less important than expected.
authors.affiliation: ERA-EDTA Registry, Department of Medical Informatics, J1B-125, Academic Medical Center, PO Box 22700, 1100 DE Amsterdam, The Netherlands.
authors.forename: Jeannette G
authors.initials: JG
authors.lastname: van Manen
authors.forename: Paul C W
authors.initials: PC
authors.lastname: van Dijk
authors.forename: Vianda S
authors.initials: VS
authors.lastname: Stel
authors.forename: Friedo W
authors.initials: FW
authors.lastname: Dekker
authors.forename: Montse
authors.initials: M
authors.lastname: Clèries
authors.forename: Ferruccio
authors.initials: F
authors.lastname: Conte
authors.forename: Terry
authors.initials: T
authors.lastname: Feest
authors.forename: Reinhard
authors.initials: R
authors.lastname: Kramar
authors.forename: Torbjørn
authors.initials: T
authors.lastname: Leivestad
authors.forename: J Douglas
authors.initials: JD
authors.lastname: Briggs
authors.forename: Bénédicte
authors.initials: B
authors.lastname: Stengel
authors.forename: Kitty J
authors.initials: KJ
authors.lastname: Jager
mesh_terms: D000367:Age Factors; D000368:Aged; D015897:Comorbidity; D003956:Dialysis; D005260:Female; D006801:Humans; D007674:Kidney Diseases / Q000150:complications*; D008297:Male; D008875:Middle Aged; D016016:Proportional Hazards Models; D017582:Renal Replacement Therapy / Q000379:methods*; D016019:Survival Analysis