Cost-utility analysis uses utility indexes to assess treatment effects. Some discrepancies between different utility indexes instruments are suggested and need to be identified in health conditions not yet investigated.
To compare different utility indexes instruments in Brazilian patients with fibromyalgia and identify variables associated with these instruments.
Impact of fibromyalgia (Fibromyalgia Impact Questionnaire [FIQ]) and utility indexes (Short-Form 6 Dimensions [SF-6D], EuroQol 5 Dimensions [EQ-5D], and EuroQol - Visual Analogue Scale [EQ-VAS]) were assessed in 97 patients with fibromyalgia at baseline, 8-week (after an exercise-based intervention), and 6- and 12-month follow-up. Construct validity and responsiveness of the utility indexes instruments were compared. Multiple regression models were used to verify the variables associated with the utility indexes instruments.
Construct validity analysis showed that FIQ presented moderate correlation with the SF-6D, the EQ-5D, and the EQ-VAS (r=-0.43, -0.41, -0.30, respectively, all p < .01). There was a moderate correlation between the SF-6D and the EQ-5D (r = 0.51, p < .001), moderate correlation between the SF-6D and the EQ-VAS (r = 0.41, p < .001), and no correlation between the EQ-VAS and the EQ-5D. The EQ-5D was responsive at the 8-week and 6-month follow-up, the SF-6D was responsive only at 6-month follow-up and the EQ-VAS was not responsive. The FIQ was associated with the EQ-5D and the SF-6D indexes, and symptom duration and depression with the EQ-VAS index.
The EQ-5D better assessed the clinical change in patients with fibromyalgia. Furthermore, impact of fibromyalgia, symptom duration, and depression seem to be associated with the utility indexes.