Purpose: To compare aspects of validity of EuroQol-5D (EQ-5D) and SF-6D, two indirect utility instruments, and the well-being rating scale (RS) in ankylosing spondylitis (AS).
Methods: EQ-5D, SF-6D and RS were available for 254 patients fulfilling modified New York criteria. 134 patients were part of an observational cohort and 120 of a randomized controlled trial (RCT). Aspects of validity assessed were truth (agreement and correlation with external health measures) and discrimination (between health states, repeatability and detection of treatment effect).
Results: Median (range) was 0.69 (-0.08-1.00) for EQ-5D, 0.65 (0.35-0.95) for the SF-6D and 0.65 (0.14- 1.00) for the RS. Agreement (ICC) was moderate (0.46 to 0.55). Instruments correlated equally with disease activity, functioning and quality of life. The SF-6D showed smaller average differences in utility between patients with better and worse disease compared to the EQ-5D and RS. The smallest detectable difference (in control group of RCT) was 0.36, 0.17 and 0.33 for EQ-5D, SF-6D and RS respectively. The ability to detect treatment effect (in intervention group of RCT) showed standardised effect sizes that were moderate for EQ-5D and SF-6D (0.63 and 0.64) and low for the RS (0.23).
Conclusion: In patients with AS, EQ-5D, SF-6D and the RS correlate equally well with external measures of health but have different psychometric properties. The smallest detectable difference is most favourable for the SF-6D, but it discriminates less well between patients with different disease severity. The RS has worse ability to detect treatment effects. It is difficult to recommend one of the instruments.
- ankylosing spondylitis
- health related quality of life
- quality adjusted life years