Purpose: Mapping is a widely used method to convert scores from condition-specific instruments to utility values. There is limited knowledge of the extent to which use of different generic preference-based measures for mapping gives rise to differences in estimates of cost-effectiveness. We compared estimates of cost-effectiveness derived from two different generic preference-based measures for a proposed therapy in the management of knee osteoarthritis.
Method: An economic model was used to estimate cost-effectiveness of glucosamine sulphate therapy for knee osteoarthritis, using evidence from a systematic review of randomized controlled trials of clinical effectiveness, together with data from observational studies. The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) was the condition-specific instrument used in clinical effectiveness trials, and scores were converted to utility weights using published mapping algorithms for the Health Utilities Index Mark 3 (HUI3) and the EQ-5D measures. The mapped values were then used to estimate quality of life gains associated with therapy, and to predict cost-effectiveness relative to current care.
Result: Using the HUI3 mapping algorithm, trials showed that therapy was associated with an annual quality of life gain of 0.005. Taking a lifetime horizon, the incremental cost per quality adjusted life year (QALY) gain for adding glucosamine sulphate to current care was approximately £21,000. At a cost per QALY gain threshold of £20,000, the likelihood that glucosamine sulphate was more cost-effective than current care was 0.43, whilst at a threshold of £30,000, the probability rose to 0.73. Using the EQ-5D mapping algorithm however, therapy was associated with an annual quality of life loss of 0.006. In this situation, current care was predicted to be more cost-effective than therapy at standard cost-effectiveness thresholds.
Conclusion: Cost-effectiveness estimates were highly sensitive to the choice of generic-based measure used in mapping. This suggests that future studies should be encouraged to use generic-preference based instruments in the first instance rather than rely on mapping as a means to estimate cost-effectiveness.
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