The aim of this study was to perform a psychometric analysis of the Italian Fibromyalgia Impact Questionnaire Revised version (FIQR), using both classical test theory (CTT) and Rasch analysis (RA) in order to better analyse its construct validity and provide a rational basis for a possible improvement of its metric quality.
The study involved 503 patients with fibromyalgia (FM) (423 women and 80 men) with mean age of 51.3±10.1 years (range 19-74) and mean duration of symptoms of 11.1±8.7 years (range 1-30). All patients completed the Italian FIQR during their clinical visit. The translation and cultural adaptation process of the Italian FIQR followed the published guidelines and no local adjustments were made except for a slight adaptation of item 13 related to 'energy'.
Factor Analysis revealed two salient dimensions: function (items 1-9) and symptoms (items 12-21). RA was thus performed on these two subscales. Rating scale diagnostics suggested collapsing the eleven rating categories of the scale into five. After combining these rating categories, RA showed that most items of each of the two subscales fitted the respective constructs to measure (MnSq 0.7-1.3). The reliability levels of the two subscales were higher than 0.80.
This study provides psychometric evidence of the reliability, internal validity and two-dimensional structure of the FIQR in a FM population. Our results support the use of two separate subscales for 'function' and 'symptoms', and provide a useful starting point for further refinement of the scale.