title: Development of a multimorbidity index: Impact on quality of life using a rheumatoid arthritis cohort.
abstract: OBJECTIVE
  To develop a multimorbidity index (MMI) based on health-related quality of life (HRQol).
  
  
  METHODS
  The index was developed in an observational RA cohort. In all, 40 morbidities recommended as core were identified using ICD-9 codes. MMIs of two types were calculated: one by enumerating morbidities (MMI.count) and the other by weighting morbidities based on their association with HRQol as assessed by EQ-5D in multiple linear regression analysis (using β-coefficients; MMI.weight). MMIs were compared to the Charlson comorbidity index (CCI) and externally validated in an international RA cohort (COMORA Study).
  
  
  RESULTS
  In all, 544 out of 876 patients were multimorbid. MMI.count was in the range 1-16 (median = 2) and MMI.weight in the range 0-38 (median = 1). Both indices were more strongly associated with EQ-5D than CCI (Spearman: MMI.count = -0.20, MMI.weight = -0.26, and CCI = -0.10; p < 0.01). R(2) obtained by linear regression using EQ-5D as a dependent variable and various indices as independent variables, adjusted for age and gender, was the highest for MMI (R(2): MMI.count = 0.05, MMI.weight = 0.11, and CCI = 0.02). When accounting for clinical disease activity index (CDAI) R(2) increased: MMI.count = 0.18, MMI.weight = 0.22, and CCI = 0.17, still showing higher values of MMI compared with CCI. External validation in different RA cohorts (COMORA, n = 3864) showed good performance of both indices (linear regression including age, gender, and disease activity R(2) = 0.30 for both MMIs).
  
  
  CONCLUSIONS
  In our cohort, MMI based on EQ-5D performed better than did CCI. Findings were reproducible in another large RA cohort. Not much improvement was gained by weighting; therefore a simple counted index could be useful to control for the effect of multimorbidity on patient's overall well-being.
authors.affiliation: Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, 75 Francis St, Boston MA, 02115; Division of Rheumatology, Department of Internal Medicine III, Medical University Vienna, Vienna, Austria. Electronic address: hradner@partners.org.
authors.forename: Helga
authors.initials: H
authors.lastname: Radner
authors.affiliation: Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, 75 Francis St, Boston MA, 02115; Department of Epidemiology, Harvard T. H. Chan School of Public Health, Boston, MA.
authors.forename: Kazuki
authors.initials: K
authors.lastname: Yoshida
authors.affiliation: Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, 75 Francis St, Boston MA, 02115; Department of Rheumatology, Diakonhjemmet Hospital, Oslo, Norway.
authors.forename: Maria Dahl
authors.initials: MD
authors.lastname: Mjaavatten
authors.affiliation: Division of Rheumatology, Department of Internal Medicine III, Medical University Vienna, Vienna, Austria.
authors.forename: Daniel
authors.initials: D
authors.lastname: Aletaha
authors.affiliation: Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, 75 Francis St, Boston MA, 02115.
authors.forename: Michelle
authors.initials: M
authors.lastname: Frits
authors.affiliation: Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, 75 Francis St, Boston MA, 02115.
authors.forename: Bing
authors.initials: B
authors.lastname: Lu
authors.affiliation: Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, 75 Francis St, Boston MA, 02115.
authors.forename: Christine
authors.initials: C
authors.lastname: Iannaccone
authors.affiliation: Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, 75 Francis St, Boston MA, 02115.
authors.forename: Nancy
authors.initials: N
authors.lastname: Shadick
authors.affiliation: Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, 75 Francis St, Boston MA, 02115.
authors.forename: Michael
authors.initials: M
authors.lastname: Weinblatt
authors.affiliation: Faculty of Medicine, Laboratory of Biostatistics, Clinical Research and Epidemiology (LBRCE), Mohammed V Souissi University, Rabat, Morocco.
authors.forename: Ihsane
authors.initials: I
authors.lastname: Hmamouchi
authors.affiliation: Department of Rheumatology, Paris Descartes University-Hôpital Cochin, Assistance Publique-Hôpitaux de Paris-EULAR center of excellence, INSERM (U1153) Clinical epidemiology and biostatistics, PRES Sorbonne Paris-Cité, Paris, France.
authors.forename: M
authors.initials: M
authors.lastname: Dougados
authors.affiliation: Division of Rheumatology, Department of Internal Medicine III, Medical University Vienna, Vienna, Austria.
authors.forename: Josef S
authors.initials: JS
authors.lastname: Smolen
authors.affiliation: Division of Rheumatology, Immunology and Allergy, Brigham and Women's Hospital, 75 Francis St, Boston MA, 02115.
authors.forename: Daniel H
authors.initials: DH
authors.lastname: Solomon
mesh_terms: D000328:Adult; D000368:Aged; D001172:Arthritis, Rheumatoid / Q000175:diagnosis* / Q000523:psychology; D005260:Female; D006304:Health Status; D006801:Humans; D008297:Male; D008875:Middle Aged; D011788:Quality of Life / Q000523:psychology*; D019057:Quality-Adjusted Life Years; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires
keywords: Comorbidity; Health-related quality of life; Multimorbidity; Rheumatoid arthritis