Making what counts be counted: evaluating the use of preference-based outcome measures in Parkinson’s disease

Parkinson’s is a common neurodegenerative disorder that can have a significant impact on an individual’s health, quality of life (QoL), and wellbeing, causing a substantial economic burden on patients, their caregivers, the health service, and broader social and community services. Whilst Parkinson’s wide range of QoL and financial impacts have been well documented relatively little research has explored to what extent such impacts have been appropriately incorporated into economic evaluations. Economic evaluation is used by the National Institute for Health and Care Excellence (NICE) in the UK to guide health-care resource allocation in the NHS. It uses preference-based outcome measures to measure and value the health outcomes of different interventions. These health utilities are combined with durations to estimate quality-adjusted life-years. The important role of the preference-based outcomes requires them to be accurately capturing the benefit of interventions, otherwise the estimation of cost-effectiveness of interventions will be not be reflecting true preference/choice between interventions. This may lead to mistakes in funding decisions and insufficient allocation of resources. Despite the importance of accurately capturing the benefit of interventions, the existing generic preference-based measures (e.g., the EQ-5D measure as recommended by NICE) are sometimes criticised for their ‘health-related’ nature as insufficient to capture all the QoL aspects that are affected by the disease or the intervention. This raises a question of “is the generic ‘prescribed’ measure appropriate for all disease areas and all interventions? ” For diseases that have broad impact on people’s health and wellbeing such as Parkinson’s, a broadly scoped preference-based wellbeing instrument which could measure the impact of intervention beyond health may potentially fill the gap of the limited scope of the ‘health-related’ preference-based measures (if any). Meanwhile, there are concerns relating to their relevance and sensitivity to specific health aspects and their validity in general to be used in the healthcare context. Given this, the aim of this thesis is to examine the performance of the existing preference-based outcomes in people with Parkinson’s, and evaluate the potential of using a generic preference-based capability-wellbeing measure, ICECAP-O, in this population. This thesis conducted a systematic review of the existing preference-based measures to assess their construct validity and responsiveness in people with Parkinson’s. Two empirical studies explored these properties of the ICECAP-O measure in people with Parkinson’s. Construct validity and responsiveness are the two key psychometric properties relevant to preference-based measures for their use in economic evaluations. Data for both empirical analyses were obtained from the PD MED large-scale randomised controlled trial. This thesis has identified evidence of limited responsiveness of the existing preference-based measures in people with Parkinson’s and suggested that the current commonly used preference-based health-related QoL measures may underestimate the value placed on the mental and social wellbeing aspects that Parkinson’s populations are affected by. This limited ability of the utility values to differentiate health states may have an impact on resource allocation decisions. Especially as this relates to the cost-effectiveness of interventions that have the capacity to influence the mental and social wellbeing aspects of people’s lives. This highlights the need for consideration of a broadly scoped measure such as the ICECAP-O to incorporate such aspects in economic evaluations of diseases such as Parkinson’s. This thesis established the construct validity and responsiveness of the ICECAP-O instrument and demonstrated that there are valued capability wellbeing attributes in Parkinson’s beyond those quality of life attributes reflected by the EQ-5D instrument. It contributes to understanding the use of broadly scoped outcome measures for economic evaluations in Parkinson’s by showing that the ICECAP-O capability wellbeing instrument was able to provide a preference-based assessment of these under-represented aspects in the Parkinson’s population, without compromising its sensitivity to the clinical and specific physical QoL dimensions in this patient group. While further exploration of the role of ICECAP-O in economic evaluation and decision making through the work of assessing ‘sufficient capability’ is required, this thesis establishes initial foundations for the use of the ICECAP-O as a preference-based instrument to measure the impact of interventions in Parkinson’s populations.