title: How to adjust for comorbidity in survival studies in ESRD patients: a comparison of different indices.
abstract: BACKGROUND
  Many patients with end-stage renal disease (ESRD) have additional comorbid conditions. Differences in the presence and severity of these comorbid conditions can bias comparisons between treatment groups. Adjustment for prognostic factors can statistically counterbalance these differences. For this purpose, appropriate weighting of comorbid conditions is necessary. We evaluated three existing methods to score comorbidity in patients with ESRD and compared their ability to predict survival: the Khan, Davies, and Charlson indices. In addition, these three indices were compared with a new index that explicitly incorporates the severity grading of a number of comorbid diseases.
  
  
  METHODS
  In a large Dutch prospective multicenter study (Netherlands Co-operative Study on the Adequacy of Dialysis-2), new patients with ESRD were included. Comorbidity was assessed at the start of dialysis therapy. Patient data were randomly allocated to a modeling or testing set. The new index was developed in the modeling set. All indices were evaluated in the testing set.
  
  
  RESULTS
  We obtained data for 1,205 patients. Of the three existing indices, the Charlson index had the best discriminating features, with a concordance c statistic of 0.71. The addition of severity grading of several comorbid conditions did not improve discrimination. After combining the comorbidity indices with age, all c statistics improved. These final values ranged from 0.72 to 0.75.
  
  
  CONCLUSIONS
  We conclude that the Khan, Davies, and Charlson scores are appropriate for expressing the prognostic impact of comorbidity on mortality risk in patients with ESRD provided sufficient adjustment for age is performed. Adding the severity grading of several comorbid conditions will not lead to improved prognostic power.
authors.affiliation: Department of Clinical Epidemiology, Dianet Dialysis Centres, University of Amsterdam, The Netherlands. j.g.vanmanen@amc.uva.nl
authors.forename: Jeannette G
authors.initials: JG
authors.lastname: van Manen
authors.forename: Johanna C
authors.initials: JC
authors.lastname: Korevaar
authors.forename: Friedo W
authors.initials: FW
authors.lastname: Dekker
authors.forename: Elisabeth W
authors.initials: EW
authors.lastname: Boeschoten
authors.forename: Patrick M M
authors.initials: PM
authors.lastname: Bossuyt
authors.forename: Raymond T
authors.initials: RT
authors.lastname: Krediet
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015331:Cohort Studies; D015897:Comorbidity / Q000639:trends; D003627:Data Interpretation, Statistical; D005260:Female; D006801:Humans; D007676:Kidney Failure, Chronic / Q000453:epidemiology*; D008297:Male; D008875:Middle Aged; D015337:Multicenter Studies as Topic / Q000379:methods / Q000706:statistics & numerical data; D011379:Prognosis; D011446:Prospective Studies; D012720:Severity of Illness Index; D016019:Survival Analysis