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How do the EQ-5D, SF-6D and the well-being rating scale compare in patients with ankylosing spondylitis?

Annelies Boonen¹, Désirée van der Heijde¹, Robert Landewé¹, Astrid van Tubergen¹, Herman Mielants², Maxime Dougados³, Sjef van der Linden¹

¹ Department of Internal Medicine, Division of Rheumatology, Caphri Research Institute, University Hospital Maastricht, Maastricht, The Netherlands
² Department of Rheumatology, University Hospital Gent, Gent Belgium
³ Department of Rheumatology, Université Réné Descartes, Hôpital Cochin, Paris, France

Correspondence to:
Dr A Boonen
Department of Internal Medicine, Division of Rheumatology, University Hospital Maastricht, PO Box 5800, 6202 AZ Maastricht, The Netherlands;aboo{at}sint.azm.nl

Purpose: To compare aspects of validity of EuroQol—5 Dimensions (EQ-5D) and Short-Form—6 Dimensions (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 were part of a randomised controlled trial (RCT). Aspects of validity assessed were truth (agreement and correlation with external health measures) and discrimination (differentiation between health states, repeatability and detection of treatment effect).

Results: Median (range) values were 0.69 (–0.08–1.00) for the EQ-5D, 0.65 (0.35–0.95) for the SF-6D and 0.65 (0.14–1.00) for the RS. Agreement (intraclass correlation coefficient) was moderate (0.46–0.55). 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 with the EQ-5D and the RS. The smallest detectable difference (SDD) (in the 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 the intervention trial) 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 SDD is most favourable for the SF-6D, but it discriminates less well between patients with different disease severities. The RS has a poorer ability to detect treatment effects. It is difficult to recommend one of the instruments.

Abbreviations: ASQoL, Ankylosing Spondylitis Quality of Life; BASDAI, Bath Ankylosing Spondylitis Disease Activity Index; BASFI, Bath Ankylosing Spondylitis Functional Index; EQ-5D, EuroQol—5 Dimensions; ICC, intraclass correlation coefficient; QALY, quality-adjusted life years; QoL, quality of life; RS, rating scale; RTE, relative treatment effect; SDD, smallest detectable difference; SES, standardised effect size; SF-36, Short-Form-36; SF-6D, Short-Form—6 Dimensions; SG, standard gamble; TTO, time-trade-off

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