The validity of the Danish version of the Fibromyalgia Impact Questionnaire - Revised applied in a clinical setting: a Rasch analysis

OBJECTIVE: The aim of this study was to evaluate the psychometric properties of the Danish version of the Fibromyalgia Impact Questionnaire - Revised (FIQR), when used to quantify the severity of disease burden in a Danish population of patients with chronic widespread pain (CWP), including fibromyalgia (FM).

METHOD: A total of 924 participants diagnosed with CWP and/or FM completed an electronic version of the FIQR via touchscreens in the clinic at referral for specialist care. Data were collected from 1 January 2018 to 3 September 2020. Rasch measurement methods were applied.

RESULTS: Rating scale analysis suggested multiple threshold disordering in the 0-10 category rating scale. A principal component analysis suggested assessment of a multidimensional construct. Thus, the Rasch analysis of the full FIQR was discontinued. Instead, Rasch analyses were performed on the two subscales: 'function' and 'symptoms'. By collapsing the rating scale to a 0-4 category scale, the remaining threshold disordering of both subscale was solved. Only the symptom subscale indicated multidimensionality. There was underfitting misfit of item 21 and overfitting misfit of item 12. No significant differential item functioning, defined by sex, ethnicity, or education, was found.

CONCLUSION: The FIQR should be considered as an instrument consisting of three separate subscales representing 'function', 'overall impact', and 'symptoms'. We recommend calculating and reporting on both a 0-10 and a 0-4 category scale. Also, if using the total FIQR score as an outcome measure, this should be done with caution, until revision of the rating scale.