title: Measuring functioning in people with fibromyalgia (FM) based on the international classification of functioning, disability and health (ICF)--a psychometric analysis.
abstract: OBJECTIVE
  Instruments to assess functioning in patients with FM vary considerably in their content and are often symptom-specific. This study aimed to examine whether it is feasible to construct a psychometric-sound clinical instrument to measure functioning in FM based on the Brief ICF-Core-Set for chronic widespread pain (CWP).
  
  
  METHODS
  Two hundred and fifty six people with FM completed the Brief ICF-Core-Set. The Rasch model was used for analysis. Once ordering of response options of ICF categories was ensured, the following properties were studied: fit of the ICF categories to the Rasch model, the targeting between ICF categories and a person's abilities, unidimensionality, and reliability.
  
  
  RESULTS
  Six ICF categories were rescored due to disordered thresholds. Five ICF categories were removed due to high model-misfit and differential item functioning (DIF) for gender. Scores from 46 participants were excluded due to extreme scores. The ICF categories included display consistency with an underlying unidimensional construct, are free of DIF for age, disease duration and gender, display excellent overall reliability, and cover a range of functioning difficulties.
  
  
  CONCLUSIONS
  This study illustrates that it is possible to measure functioning as a unidimensional construct based on selected ICF categories from the components body functions, as well as activities and participation of the Brief ICF-Core-Set for CWP in patients with FM.
authors.affiliation: Clinic for Internal Medicine III, Department of Rheumatology, Medical University of Vienna, Austria. birgit.prodinger@meduniwien.ac.at
authors.forename: Birgit
authors.initials: B
authors.lastname: Prodinger
authors.forename: Thomas
authors.initials: T
authors.lastname: Salzberger
authors.forename: Gerold
authors.initials: G
authors.lastname: Stucki
authors.forename: Tanja
authors.initials: T
authors.lastname: Stamm
authors.forename: Alarcos
authors.initials: A
authors.lastname: Cieza
mesh_terms: D000203:Activities of Daily Living / Q000145:classification*; D000328:Adult; D004185:Disability Evaluation; D005260:Female; D005356:Fibromyalgia / Q000175:diagnosis* / Q000453:epidemiology / Q000503:physiopathology*; D006305:Health Status Indicators; D006801:Humans; D038801:International Classification of Diseases / Q000592:standards*; D008297:Male; D008875:Middle Aged; D011594:Psychometrics