title: Modelling the relationship between the WOMAC Osteoarthritis Index and EQ-5D.
abstract: OBJECTIVE
  Economic evaluation typically is conducted using health state utilities to estimate treatment benefits. However, such outcomes are often missing from studies of clinical effectiveness. This study aims to bridge that gap by providing appropriate methods to link values from the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) to the EQ-5D utility instrument.
  
  
  METHODS
  Patients from a large registry of Spanish patients (n = 7072 observations) with knee or hip osteoarthritis who completed both WOMAC and EQ-5D was used. A mixture model approach was used based on distributions bespoke to the EQ-5D UK value set to estimate EQ-5D as a function of WOMAC pain, stiffness and function subscores.
  
  
  RESULTS
  A five class mixture model provides very close fit to the observed data at all levels of disease severity. The overall mean (0.542 vs 0.542), median (0.620 vs 0.636) and the percentage of observations at full health (15 vs 14.8) were very similar between the observed data and the estimated model respectively. Stiffness has limited relationship to EQ-5D, whereas functional disability and pain are strong predictors.
  
  
  CONCLUSIONS
  EQ-5D can be reliably estimated from WOMAC subscale scores without any systematic bias using these results.
authors.affiliation: Health Economics and Decision Science, School of Health and Related Research (ScHARR), University of Sheffield, Sheffield, UK. a.j.wailoo@sheffield.ac.uk.
authors.forename: Allan
authors.initials: A
authors.lastname: Wailoo
authors.forename: Monica
authors.initials: M
authors.lastname: Hernandez Alava
authors.forename: Antonio
authors.initials: A
authors.lastname: Escobar Martinez
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D016018:Least-Squares Analysis; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D015207:Osteoarthritis, Hip / Q000175:diagnosis* / Q000523:psychology; D020370:Osteoarthritis, Knee / Q000175:diagnosis* / Q000523:psychology; D017063:Outcome Assessment, Health Care; D010147:Pain Measurement; D011594:Psychometrics; D011788:Quality of Life; D019057:Quality-Adjusted Life Years; D012720:Severity of Illness Index; D012846:Singapore