Introduction: The Fibromyalgia Impact Questionnaire (FIQ) is a composite disease-specific measure validated for fibromyalgia (FM), but it is rarely used in clinical practice. The objective was to develop and analyse the psychometric properties of a new composite disease-specific index (Fibromyalgia Assessment Status, FAS), a simple self-administered index that combines a patient's assessment of fatigue, sleep disturbances and pain evaluated on the basis of the 16 non-articular sites listed on the Self-Assessment Pain Scale (SAPS) in a single measure (range 0 to 10).
Methods: The FAS index was constructed using a traditional development strategy, and its psychometric properties were tested in 226 FM patients (209 women, 17 men); whose disease-related characteristics were assessed by means of an 11-numbered circular numerical rating scale (NRS) for pain, fatigue, sleep disturbances and general health (GH), the tender point score (TPS), the SAPS, the FIQ, and the SF-36. A group of 226 rheumatoid arthritis (RA) patients was used for comparative purposes. Of the 179 FM patients who entered the follow-up study, 152 completed the three-month period and were included in the responsiveness analyses. One hundred and fifty-four patients repeated the FAS questionnaire after an interval of one week, and its test/re-test reliability was calculated. Responsiveness was evaluated on the basis of effect size and the standardised response mean.
Results: The FAS index fulfilled the established criteria for validity, reliability and responsiveness. Factor analysis showed that SAPS and fatigue contributed most, and respectively explained 47.4% and 31.2% of the variance; sleep explained 21.3%. Testing for internal consistency showed that Cronbach's alpha was 0.781, thus indicating a high level of reliability. As expected, closer significant correlations were found when FAS was compared with total FIQ (rho = 0.347; P < 0.0001) and the FIQ subscales, particularly job ability, tiredness, fatigue and pain (all P < 0.0001), but the correlation between FAS and the mental component summary scale score (MCS) of the SF-36 (rho = -0.531; P < 0.0001) was particularly interesting. Test/re-test reliability was satisfactory. The FAS showed the greatest effect size. The magnitude of the responsiveness measures was statistically different between FAS (0.889) and the FIQ (0.781) (P = 0.038), and between the SF-36 MCS (0.434) and the SF-36 physical component summary scale score (PCS) (0.321) (P < 0.01).
Conclusions: The self-administered FAS is a reliable, valid and responsive disease-specific composite measure for assessing treatment effect in patients with FM.