• Birgit Prodinger, Thomas Salzberger, Gerold Stucki, Tanja Stamm, and Alarcos Cieza.
• Clinic for Internal Medicine III, Department of Rheumatology, Medical University of Vienna, Austria. birgit.prodinger@meduniwien.ac.at
• Pain Pract. 2012 Apr 1; 12 (4): 255-65.

ObjectivesInstruments 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).MethodsTwo 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.ResultsSix 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.ConclusionsThis 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.© 2011 The Authors. Pain Practice © 2011 World Institute of Pain.

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