Psychometric evaluation of the Norwegian version of the Revised Fibromyalgia Impact Questionnaire

The Revised Fibromyalgia Impact Questionnaire (FIQ-R) is a composite questionnaire covering key domains of fibromyalgia disease burden. This study aimed to translate the FIQ-R into Norwegian, to assess face, content, and criterion validity, and to evaluate the psychometric properties of the Norwegian FIQ-R using Rasch analysis, in a sample of adults with fibromyalgia.

The FIQ-R was translated into Norwegian following established methodology in collaboration with patient research partners. Participants attending a patient education session for people with chronic musculoskeletal pain were asked to complete, anonymously, the translated FIQ-R, Fibromyalgia Self-reported Diagnostic Criteria (FSDC), 36-item Short Form Health Survey (SF-36), and Modified Health Assessment Questionnaire (MHAQ). Fibromyalgia cases were identified according to the FSDC. Correlations between FIQ-R, MHAQ, and SF-36 were calculated. Rasch analysis was performed.

In total, 241 participants (96.3% female) were classified as having fibromyalgia according to the FSDC. The translated FIQ-R had acceptable face and content validity. The correlation coefficients were moderate between FIQ-R and SF-36, and strong between FIQ-R and MHAQ. The FIQ-R formed a multidimensional scale, indicating two subscales: FIQ-R function and FIQ-R symptoms. Both subscales showed reversed threshold ordering and were consequently collapsed into five and four response categories, respectively. The resulting subscales were found to have good reliability and targeting.

Owing to the lack of unidimensionality in the FIQ-R, it is recommended that both clinical and research applications rely on subscale scores rather than a total score.