title: Validation of the rheumatic disease comorbidity index.
abstract: OBJECTIVE
  There is no consensus on which comorbidity index is optimal for rheumatic health outcomes research. We compared a new Rheumatic Disease Comorbidity Index (RDCI) with the Charlson-Deyo Index (CDI), Functional Comorbidity Index (FCI), Elixhauser Total Score (ETS), Elixhauser Point System (EPS), and a simple comorbidity count (COUNT) using a US cohort of rheumatoid arthritis (RA) patients.
  
  
  METHODS
  Using administrative diagnostic codes and patient self-reporting, we tested predictive values of the RDCI, CDI, FCI, ETS, EPS, and COUNT for 2 outcomes: all-cause mortality and physical functioning. Indices were compared using 3 models: bare (consisting of age, sex, and race), administrative (bare plus visit frequency, body mass index, and treatments), and clinic (administrative plus erythrocyte sedimentation rate, nodules, rheumatoid factor positivity, and patient activity scale).
  
  
  RESULTS
  The ETS and RDCI best predicted death, with FCI performing the worst. The FCI best predicted function, with ETS and RDCI performing nearly as well. CDI predicted function poorly. The order of indices remained relatively unchanged in the different models, though the magnitude of improvement in Akaike's information criterion decreased in the administrative and clinic models.
  
  
  CONCLUSIONS
  The RDCI and ETS are excellent indices as a means of accounting for comorbid illness when the RA-related outcomes of death and physical functioning are studied using administrative data. The RDCI is a versatile index and appears to perform well with self-report data as well as administrative data. Further studies are warranted to compare these indices using other outcomes in diverse study populations.
authors.affiliation: Omaha Veterans Affairs Medical Center and University of Nebraska Medical Center, Omaha.
authors.forename: Bryant R
authors.initials: BR
authors.lastname: England
authors.affiliation: Omaha Veterans Affairs Medical Center and University of Nebraska Medical Center, Omaha.
authors.forename: Harlan
authors.initials: H
authors.lastname: Sayles
authors.affiliation: Omaha Veterans Affairs Medical Center and University of Nebraska Medical Center, Omaha.
authors.forename: Ted R
authors.initials: TR
authors.lastname: Mikuls
authors.affiliation: G. V. Sonny Montgomery Veterans Affairs Medical Center and University of Mississippi, Jackson.
authors.forename: Dannette S
authors.initials: DS
authors.lastname: Johnson
authors.affiliation: Omaha Veterans Affairs Medical Center and University of Nebraska Medical Center, Omaha, and National Data Bank for Rheumatic Diseases, Wichita, Kansas.
authors.forename: Kaleb
authors.initials: K
authors.lastname: Michaud
mesh_terms: D000368:Aged; D015897:Comorbidity; D017281:Cost of Illness; D003661:Decision Support Techniques; D004185:Disability Evaluation; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D011379:Prognosis; D012042:Registries; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012216:Rheumatic Diseases / Q000175:diagnosis* / Q000401:mortality / Q000503:physiopathology / Q000523:psychology; D018570:Risk Assessment; D012307:Risk Factors; D013997:Time Factors