title: Standardizing disease-specific quality of life measures across multiple chronic conditions: development and initial evaluation of the QOL Disease Impact Scale (QDIS®).
abstract: BACKGROUND
  To document the development and evaluation of the Quality of life Disease Impact Scale (QDIS®), a measure that standardizes item content and scoring across chronic conditions and provides a summary, norm-based QOL impact score for each disease.
  
  
  METHODS
  A bank of 49 disease impact items was constructed from previously-used descriptions of health impact to represent ten frequently-measured quality of life (QOL) content areas and operational definitions successfully utilized in generic QOL surveys. In contrast to health in general, all items were administered with attribution to a specific disease (osteoarthritis, rheumatoid arthritis, angina, myocardial infarction, congestive heart failure, chronic kidney disease (CKD), diabetes, asthma, or COPD). Responses from 5418 adults were analyzed as five disease groups: arthritis, cardiovascular, CKD, diabetes, and respiratory. Unidimensionality, item parameter and scale-level invariance, reliability, validity and responsiveness to change during 9-month follow-up were evaluated by disease group and for all groups combined using multi-group confirmatory factor analysis (MGCFA), item response theory (IRT) and analysis of variance methods. QDIS was normed in an independent chronically ill US population sample (N = 4120).
  
  
  RESULTS
  MGCFA confirmed a 1-factor model, justifying a summary score estimated using equal parameters for each item across disease groups. In support of standardized IRT-based scoring, correlations were very high between disease-specific and standardized IRT item slopes (r = 0.88-0.96), thresholds (r = 0.93-0.99) and person-level scores (r ≥ 0.99). Internal consistency, test-retest and person-level IRT reliability were consistently satisfactory across groups. In support of interpreting QDIS as a disease-specific measure, in comparison with generic measures, QDIS consistently discriminated markedly better across disease severity levels, correlated higher with other disease-specific measures in cross-sectional tests, and was more responsive in comparisons of groups with better, same or worse evaluations of disease-specific outcomes at the 9-month follow-up.
  
  
  CONCLUSIONS
  Standardization of content and scoring across diseases was shown to be justified psychometrically and enabled the first summary measure of disease-specific QOL impact normed in the chronically ill population. This disease-specific approach substantially improves discriminant validity and responsiveness over generic measures and provides a basis for better understanding the relative QOL impact of multiple chronic conditions in research and clinical practice.
authors.affiliation: John Ware Research Group, 10 Wheeler Court, Watertown, MA, 02472, USA. john.ware@jwrginc.com.|Department of Quantitative Health Sciences, University of Massachusetts Medical School, Worcester, MA, USA. john.ware@jwrginc.com.
authors.forename: John E
authors.initials: JE
authors.lastname: Ware
authors.affiliation: John Ware Research Group, 10 Wheeler Court, Watertown, MA, 02472, USA.|Department of Quantitative Health Sciences, University of Massachusetts Medical School, Worcester, MA, USA.
authors.forename: Barbara
authors.initials: B
authors.lastname: Gandek
authors.affiliation: John Ware Research Group, 10 Wheeler Court, Watertown, MA, 02472, USA.
authors.forename: Rick
authors.initials: R
authors.lastname: Guyer
authors.affiliation: Department of Quantitative Health Sciences, University of Massachusetts Medical School, Worcester, MA, USA.|Measured Progress, Dover, NH, USA.
authors.forename: Nina
authors.initials: N
authors.lastname: Deng
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000704:Analysis of Variance; D002908:Chronic Disease / Q000523:psychology*; D015897:Comorbidity; D017281:Cost of Illness; D003430:Cross-Sectional Studies; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011594:Psychometrics; D011788:Quality of Life; D015203:Reproducibility of Results; D012720:Severity of Illness Index; D055815:Young Adult
keywords: Disease-specific measures; Health-related quality of life; Item response theory; Multiple chronic conditions; Norm-based scoring; Patient-reported outcomes; Responsiveness; Validity