title: Interchangeability of the EQ-5D and the SF-6D in long-lasting low back pain.
abstract: OBJECTIVE
  The objective of this study was to investigate the interchangeability of the EuroQol 5D (EQ-5D) and the Short Form 6D (SF-6D) in individuals with long-lasting low back pain to guide the optimal choice of instrument and to inform decision-makers about any between-measure discrepancy, which require careful interpretation of the results of cost-utility evaluations.
  
  
  METHODS
  A cross-sectional study was conducted across 275 individuals who had spinal surgery on indication of chronic low back pain. EQ-5D and SF-6D were mailed to respondents for self-completion. Statistical analysis of between-measure agreement (using English weights) was based on Bland and Altman's limits of agreement and a series of linear regressions.
  
  
  RESULTS
  A moderate mean difference of 0.085 (SD 0.241) was found, but because it masked more severe bidirectional variation, the expected variation between observations of EQ-5D and SF-6D in future studies was estimated at 0.546. The EQ-5D's N3 term alone explained a factor of 0.79 of the variation in between-measure differences, while the explanatory value of adding variables of age, sex, diagnosis, previous surgery, and occupational status was basically zero. A final model including only dummy variables for the N3 term and five identified framing effects explained a factor of 0.86 of the variation in between-measure differences.
  
  
  CONCLUSIONS
  Although the EQ-5D and the SF-6D are both psychometrically valid for generic outcome assessment in long-lasting low back pain, it appears that they cannot generally be used interchangeably for measurement of preference values. Sensitivity analysis examining the impact of between-measure discrepancy thus remains a necessary condition for the interpretation of the results of cost-utility evaluations.
authors.affiliation: CAST (Centre for Applied Health Services Research and Technology Assessment), Institute for Public Health, University of Southern Denmark, Odense, Denmark. ris@cast.sdu.dk
authors.forename: Rikke
authors.initials: R
authors.lastname: Søgaard
authors.forename: Finn Bjarke
authors.initials: FB
authors.lastname: Christensen
authors.forename: Tina Senholt
authors.initials: TS
authors.lastname: Videbaek
authors.forename: Cody
authors.initials: C
authors.lastname: Bünger
authors.forename: Terkel
authors.initials: T
authors.lastname: Christiansen
mesh_terms: D000328:Adult; D000368:Aged; D003430:Cross-Sectional Studies; D003657:Decision Making; D005260:Female; D006304:Health Status; D006305:Health Status Indicators; D006801:Humans; D016014:Linear Models; D017116:Low Back Pain / Q000601:surgery; D008159:Lumbar Vertebrae / Q000601:surgery; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D017063:Outcome Assessment, Health Care / Q000379:methods* / Q000706:statistics & numerical data; D057240:Patient Preference; D011594:Psychometrics; D011788:Quality of Life; D013123:Spinal Fusion; D013997:Time Factors; D055815:Young Adult