The aim of this study was to compare three health utility instruments (15D, EQ-5D, SF-6D) and a rating scale for health (EQ-Visual Analogue Scale (VAS)) and to investigate their relationship to clinical parameters in patients with rheumatoid arthritis (RA).
Data were collected from 1041 patients with RA. Agreement between the instruments was assessed with Bland-Altman plots. Linear regression models were fitted for the different instruments and Health Assessment Questionnaire (HAQ) scores, age, gender, patient global, disease duration and educational level. Differences in utility scores across levels of global health and disability, were investigated as well as correlations with disease-specific health status measures.
The score range in the 1041 patients with RA was 0.41-1.0 for 15D, -0.48 to 1.0 for EQ-5D, 0.0-1.0 for EQ-VAS and 0.30-1.0 for SF-6D, with a bimodal distribution for EQ-5D. Bland-Altman plots indicated poor agreement between EQ-5D and SF-6D/15D and moderate agreement between SF-6D and 15D. Utility scores were correlated with disease-specific measures, pain and fatigue (r>0.60). Mean utilities ranged from 0.30 (EQ-5D) to 0.69 (15D) in patients rating their own health as poor. When correcting for a non-linear relationship between HAQ and EQ-5D/SF-6D in linear regression models, the estimated utilities had non-overlying CI for HAQ values >1.4.
Diverging scores were observed across utility instruments, especially in patients with high HAQ scores. The choice of utility instrument may have an impact on the results of cost-utility analyses, with large hypothetical differences in price per quality-adjusted life year.