To estimate the relationship between preference-based measures, EuroQol (EQ-5D) and SF-6D, and the Health Assessment Questionnaire (HAQ) disability index (DI) in patients with rheumatoid arthritis (RA), and to characterize components that are predictors of health utility.
Patients with RA participating in 2 studies in the UK (n = 151) and Canada (n = 319) completed the HAQ, EQ-5D, and Short Form 36 (SF-36). The SF-36, a generic measure of quality of life, was converted into the preference-based SF-6D. From these results we developed models of the relationship between the HAQ and SF-6D and EQ-5D using various regression analyses.
The optimal model developed for the EQ-5D entered levels for each item as independent variables (model 5). A root mean square error (RMSE) of 0.18 suggested relatively good predictive ability. For the SF-6D, RMSEs were lower (0.09), suggesting better predictions than for the EQ-5D, but models with more explanatory variables did not improve results (model 2 or 4 optimal). The models were able to predict actual SF-6D and EQ-5D across the range of the HAQ DI.
Our approach enabled calculations of quality-adjusted life years from existing trials where only the HAQ was measured. All aspects of the HAQ may not be reflected in the preference-based measures, and this method is suboptimal to direct measurement of health state utility in clinical trials. Given this limitation, our approach provides an alternative for researchers who need health-state utility values, but had not included a preference-based measure in their clinical study because of resource constraints or a desire to limit patient burden.