Functional assessment by self-report questionnaire plays an important role in most rheumatic conditions, but psychometric properties of questionnaires have not been studied in fibromyalgia (FM), particularly by Rasch analysis, which allows for examining adequacy of the questionnaire scale. To assess currently used instruments, we examined the Fibromyalgia Impact Scale (FIQ), 4 versions of the Health Assessment Questionnaire (HAQ), and the Medical Outcome Survey Short Form (SF-36).
More than 2,500 patients from 4 sites (3 US, 1 Israel) completed the FIQ. The HAQ questionnaires were completed by 1438 patients participating in the US National Data Bank for Rheumatic Diseases. Seven hundred sixty patients from Wichita, Kansas, completed the SF-36. Rasch analysis was applied separately to each of these data sets.
The FIQ systematically underestimated functional impairment by its handling of activities not usually performed. All questionnaires had problems with non-unidimensionality and ambiguous items when applied to patients with FM. In addition, scales were found to be non-linear. Because of these findings we used the 20 item HAQ questionnaire as an item bank to develop a new questionnaire more suitable for use in FM, the fibromyalgia HAQ (FHAQ). This questionnaire fits the Rasch model well, is relevant, is linear, and has a long, well spaced scale.
No available functional assessment questionnaire works well in FM. A new questionnaire, the FHAQ, was developed. It has appropriate metric properties and should function well in this condition. Since the FHAQ is a subset of the larger HAQ questionnaire, a new questionnaire is not required; only a different method of scoring is needed. Additional studies regarding sensitivity to change are required to fully validate the FHAQ.