title: Longitudinal predictive ability of mapping models: examining post-intervention EQ-5D utilities derived from baseline MHAQ data in rheumatoid arthritis patients.
abstract: OBJECTIVE
  The purpose of this methodological study was to to provide insight into the under-addressed issue of the longitudinal predictive ability of mapping models. Post-intervention predicted and reported utilities were compared, and the effect of disease severity on the observed differences was examined.
  
  
  METHODS
  A cohort of 120 rheumatoid arthritis (RA) patients (60.0% female, mean age 59.0) embarking on therapy with biological agents completed the Modified Health Assessment Questionnaire (MHAQ) and the EQ-5D at baseline, and at 3, 6 and 12 months post-intervention. OLS regression produced a mapping equation to estimate post-intervention EQ-5D utilities from baseline MHAQ data. Predicted and reported utilities were compared with t test, and the prediction error was modeled, using fixed effects, in terms of covariates such as age, gender, time, disease duration, treatment, RF, DAS28 score, predicted and reported EQ-5D.
  
  
  RESULTS
  The OLS model (RMSE = 0.207, R(2) = 45.2%) consistently underestimated future utilities, with a mean prediction error of 6.5%. Mean absolute differences between reported and predicted EQ-5D utilities at 3, 6 and 12 months exceeded the typically reported MID of the EQ-5D (0.03). According to the fixed-effects model, time, lower predicted EQ-5D and higher DAS28 scores had a significant impact on prediction errors, which appeared increasingly negative for lower reported EQ-5D scores, i.e., predicted utilities tended to be lower than reported ones in more severe health states.
  
  
  CONCLUSIONS
  This study builds upon existing research having demonstrated the potential usefulness of mapping disease-specific instruments onto utility measures. The specific issue of longitudinal validity is addressed, as mapping models derived from baseline patients need to be validated on post-therapy samples. The underestimation of post-treatment utilities in the present study, at least in more severe patients, warrants further research before it is prudent to conduct cost-utility analyses in the context of RA by means of the MHAQ alone.
authors.affiliation: Faculty of Social Sciences, Hellenic Open University, Patras, Greece. nkontodi@otenet.gr
authors.forename: Nick
authors.initials: N
authors.lastname: Kontodimopoulos
authors.forename: Panagiotis
authors.initials: P
authors.lastname: Bozios
authors.forename: John
authors.initials: J
authors.lastname: Yfantopoulos
authors.forename: Dimitris
authors.initials: D
authors.lastname: Niakas
mesh_terms: D018501:Antirheumatic Agents / Q000191:economics* / Q000627:therapeutic use*; D001172:Arthritis, Rheumatoid / Q000188:drug therapy*; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D018803:Models, Economic; D017063:Outcome Assessment, Health Care; D011237:Predictive Value of Tests; D011788:Quality of Life; D012044:Regression Analysis; D012720:Severity of Illness Index
chemical_list: D018501:Antirheumatic Agents