title: Mapping the FACT-P to the preference-based EQ-5D questionnaire in metastatic castration-resistant prostate cancer.
abstract: OBJECTIVE
  To develop a mapping algorithm for estimating EuroQol five-dimensional (EQ-5D) questionnaire values from the prostate cancer-specific health-related quality-of-life (HRQOL) instrument Functional Assessment of Cancer Therapy-Prostate (FACT-P) instrument.
  
  
  METHODS
  The EQ-5D questionnaire and FACT-P instrument data were collected for a subset of patients with metastatic castration-resistant prostate cancer in a multicenter, randomized, double-blind, placebo-controlled phase 3 trial. We compared three statistical techniques to estimate patients' EQ-5D questionnaire index scores determined by using the UK tariff: 1) generalized estimating equations, 2) two-part model combining logistic regression and generalized estimating equation, and 3) separate mapping algorithms for patients with poor health defined as a FACT-P score of 76 or less (group-specific model). Four different sets of explanatory variables were compared. The models were cross-validated by using a 10-fold in-sample cross-validation.
  
  
  RESULTS
  Values for both instruments were available for 236 patients with metastatic castration-resistant prostate cancer. The group-specific model including the FACT-P subscale scores and baseline variables had the best predictive performance with R(2) 0.718, root mean square error 0.162, and mean absolute error 0.117. The two-part model and the generalized estimating equation model including the FACT-P subdomain scores and baseline variables also had good predictive performance.
  
  
  CONCLUSIONS
  The developed algorithms for mapping the FACT-P instrument to the EQ-5D questionnaire enable the estimation of preference-based health-related quality-of-life scores for use in cost-effectiveness analyses when directly elicited EQ-5D questionnaire data are missing.
authors.affiliation: Quintiles, Barcelona, Spain; Faculty of Medicine, University of Barcelona, Barcelona, Spain. Electronic address: konstantina.skaltsa@quintiles.com.
authors.forename: Konstantina
authors.initials: K
authors.lastname: Skaltsa
authors.affiliation: Health Economics Research Group, Brunel University, Uxbridge, UK.
authors.forename: Louise
authors.initials: L
authors.lastname: Longworth
authors.affiliation: Quintiles, Hoofddorp, The Netherlands.
authors.forename: Cristina
authors.initials: C
authors.lastname: Ivanescu
authors.affiliation: Astellas Pharma Global Development, Leiden, The Netherlands.
authors.forename: De
authors.initials: D
authors.lastname: Phung
authors.affiliation: Astellas Pharma Global Development, Leiden, The Netherlands.
authors.forename: Stefan
authors.initials: S
authors.lastname: Holmstrom
mesh_terms: D000368:Aged; D000465:Algorithms; D004311:Double-Blind Method; D006801:Humans; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D009362:Neoplasm Metastasis; D011237:Predictive Value of Tests; D064129:Prostatic Neoplasms, Castration-Resistant / Q000473:pathology*; D011788:Quality of Life; D011795:Surveys and Questionnaires
keywords: EQ-5D; FACT-P; mapping; metastatic castration-resistant prostate cancer; quality of life