title: Content and construct validity of the Rheumatic Diseases Comorbidity Index in patients with gout.
abstract: OBJECTIVE
  Gout has been associated with a large number of co-morbidities. As yet, no co-morbidity measure has been validated for use in clinical studies in gout. This study aims to evaluate the content and construct validity of the Rheumatic Diseases Comorbidity Index (RDCI) and a gout-specifically modified RDCI (mRDCI) in patients with gout.
  
  
  METHODS
  In a cross-sectional sample of 122 patients with gout, data on co-morbidities were obtained during an interview, chart review and clinical examination. The data were used to compute the RDCI/mRDCI, a simple co-morbidity count, the Charlson Comorbidity Index (CCI) and the Functional Comorbidity Index (FCI). Content and construct validity was explored by assessing Spearman correlations between the two RDCI versions and between RDCI/mRDCI and the other co-morbidity indices, as well as demographic and clinical outcomes. In addition, we assessed the independent association between the RDCI/mRDCI and physical functioning (HAQ disability index), physical health (36-Item Short Form Health Survey) and direct health care and non-health care costs using multivariable regression analyses.
  
  
  RESULTS
  The correlation between the RDCI and mRDCI was 0.86. Correlations between the RDCI/mRDCI and simple co-morbidity count, CCI or FCI varied between 0.72 and 0.88. Correlations with generic and gout-specific health outcomes were moderate and weak, respectively, with slightly better results for the mRDCI. Multivariable analyses showed that both the RDCI and mRDCI contributed to the variation in physical functioning, physical health and direct health care and non-health care costs.
  
  
  CONCLUSIONS
  Both the RDCI and mRDCI have appropriate content and construct validity to evaluate the influence of co-morbidity on outcome in patients with gout.
authors.affiliation: Department of Internal Medicine, Division of Rheumatology, Maastricht University Medical Centre, CAPHRI Research Institute, Maastricht University, Maastricht, The Netherlands and bartholomeus.spaetgens@mumc.nl.
authors.forename: Bart
authors.initials: B
authors.lastname: Spaetgens
authors.affiliation: Department of Internal Medicine, Division of Rheumatology, Maastricht University Medical Centre, CAPHRI Research Institute, Maastricht University, Maastricht, The Netherlands and.
authors.forename: José M A
authors.initials: JM
authors.lastname: Wijnands
authors.affiliation: Department of Internal Medicine, Division of Rheumatology, Maastricht University Medical Centre, Département de Rhumatologie, Centre Hospitalier Universitaire du Sart-Tilman, Liège, Belgium.
authors.forename: Caroline
authors.initials: C
authors.lastname: van Durme
authors.affiliation: Department of Internal Medicine, Division of Rheumatology, Maastricht University Medical Centre, CAPHRI Research Institute, Maastricht University, Maastricht, The Netherlands and.
authors.forename: Annelies
authors.initials: A
authors.lastname: Boonen
mesh_terms: D000368:Aged; D015897:Comorbidity; D003430:Cross-Sectional Studies; D004185:Disability Evaluation; D005260:Female; D006073:Gout / Q000175:diagnosis / Q000453:epidemiology*; D017048:Health Care Costs; D006306:Health Surveys; D006801:Humans; D007674:Kidney Diseases / Q000175:diagnosis / Q000453:epidemiology*; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D011788:Quality of Life; D015203:Reproducibility of Results; D012216:Rheumatic Diseases; D012720:Severity of Illness Index
keywords: HRQOL; Rheumatic Diseases Comorbidity Index; co-morbidities; gout; physical function