Objectives: There is growing emphasis on the cost-effectiveness of treating rheumatoid arthritis. Few trials directly record the health utility measures, like EuroQol, needed for economic analyses. Consequently linear regression methods have been used to transform Health Assessment Questionnaire (HAQ) scores into utility measures. The authors examined whether this is justified.
Methods: The authors compared HAQ and EuroQol in cross-sectional and treatment change observational studies of rheumatoid arthritis patients; they also measured SF-36 and Nottingham Health Profiles.
Results: In the cross-sectional study, HAQ and EuroQol scores were moderately inversely correlated (Spearman rank correlation, r = 0.76). HAQ showed a Gaussian distribution whereas EuroQol was bimodal. In the treatment change study, changes in HAQ and EuroQol were unrelated (r = 0.08); the changes showed similar Gaussian and bimodal distributions.
Conclusions: Not all patient-based measures are analogous, and evidence of clinical equivalence, especially in treatment response, is needed before data transformation is considered. Specifically, as HAQ and EuroQol are demonstrably not equivalent, economic evaluations of treatment cost effectiveness should not be based on EuroQol data transformed from HAQ. The use of such transformed data by regulatory bodies which determine drug availability means that the issue is no longer only of academic interest but a real clinical concern.
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Competing interests: B Khoshaba and G H Kingsley have received no direct payments from companies involved in the evaluation or marketing of anti-rheumatic drugs used in rheumatoid arthritis in the last five years, including support to attend meetings, fees for consulting and funding for research or educational support. E H Choy and D L Scott have received clinical trial grants, unrestricted educational grants and personal sponsorship for attending meetings from several companies involved in clinical trials and marketing of anti-rheumatic drugs and biologics, together with fees for speaking at meetings and giving professional advice from Amgen, Roche and Wyeth.
Funding: We are pleased to acknowledge financial support for this study from the Arthritis Research Campaign (Programme Grant S0682 and Integrated Clinical Arthritis Centre Grant P0572) and from National Health Service R&D Support Funding to Kings College Hospital and University Hospital Lewisham.
- Health Assessment Questionnaire
- mental component summary
- Nottingham Health Profile
- physical component summary
- quality adjusted life year