Estimating a Preference-Based Index for an Eight-Dimensional Health State Classification System for Multiple Sclerosis

Background: Condition-specific measures are frequently used to assess the health-related quality of life of people with multiple sclerosis (MS). Such measures are unsuitable for use in economic evaluations that require estimates of cost per quality-adjusted life year because they are not based on preferences. Objectives: To report the estimation of a preference-based single index for an eight dimensional instrument for MS, the Multiple Sclerosis Impact Scale - Eight Dimensions (MSIS-8D), derived from an MS-specific measure of health-related quality of life, the 29-item Multiple Sclerosis Impact Scale (MSIS-29). Methods: We elicited preferences for a sample of MSIS-8D states (n = 169) from a sample (n = 1702) of the UK general population. Preferences were elicited using the time trade-off technique via an Internet-based survey. We fitted regression models to these data to estimate values for all health states described by the MSIS-8D. Estimated values were assessed against MSIS-29 scores and values derived from generic preference-based measures in a large, representative sample of people with MS. Results: Participants reported that the time trade-off questions were easy to understand. Observed health state values ranged from 0.08 to 0.89. The best performing model was a main effects, random effects model (mean absolute error = 0.04). Validation analyses support the performance of the MSIS-8D index: it correlated more strongly than did generic measures with MSIS-29 scores, and it discriminated effectively between subgroups of people with MS. Conclusions: The MSIS-8D enables health state values to be estimated from the MSIS-29, adding to the methods available to assess health outcomes and to estimate quality adjusted life-years for MS for use in health technology assessment and decision-making contexts.