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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