Mapping is not an effective solution to the problem of measuring and valuing HRQL where EQ-5D has been found to be inappropriate. There may be a preference for using the EQ-5D to maintain
consistency between analyses and, therefore, adaptations to the questionnaire may be a potential solution. We examined a new approach of bolt-ons and developed and tested three bolt-on items in an exploratory study valuing nine health states from each descriptive system. We focused on the two areas where the
EQ-5D was identified to have some problems in the literature review: hearing and vision. Furthermore,
although the review found that EQ-5D performs well in cancer, there have been concerns about the face validity regarding the lack of an energy dimension in EQ-5D and this was also included in the exploratory analysis.
All three of the bolt-on items had an impact on TTO values for the EQ + bolt-on states, but the results suggested that the relationship may not be straightforward. The extent and direction of the impact of the bolt-on varied according to the level of severity of the bolt-on and the severity of the core EQ-5D state to which it was added. In most cases, including a level 1 bolt-on resulted in no difference or higher values, the addition of level 2 was mixed and the addition of level 3 led to lower values.
The results for the tiredness bolt-on differed to those from a previous study assessing the inclusion of a
similar dimension within the EQ-5D.226However, this disagreement could be attributed to any number of
differences in study design including the method of valuation (VAS compared with TTO) and the number and labelling of the bolt-on levels. All three of the bolt-ons in the exploratory study showed some impact. There did not appear to substantial differences between the three bolt-ons, although the impact appeared to be marginally strongest for the vision bolt-on and this was selected for full valuation. However, we believe that based on the results of the exploratory analysis, the tiredness and hearing bolt-on items warrant further investigation and development.
Given the results of the exploratory study, a full valuation of the vision bolt-on was conducted using face-to-face TTO interviews with members of the general public. The results of this study show that the
vision bolt-on had a significant impact on EQ-5D state valuations. As with the exploratory analysis, the
results suggest a somewhat complex relationship between the bolt-on and EQ-5D. Health states with a level 3 (extreme) vision problems included are unsurprisingly lower than the corresponding EQ-5D health
state; however, the values given to severe EQ-5D states are higher if‘no problems’ on vision are explicitly
mentioned (EQ + vision) compared with if vision is not mentioned at all (EQ-5D only). This could be due to people focusing on the positive aspect of the health state or considering the absence of vision problems to
be‘ray of light’ in an otherwise severe health state. Some qualitative exploration of what people consider
when responding would be informative.
It would be easier and less resource-intensive if future bolt-on items could be valued separately rather than
conducting a valuation of the full bolt-on classification including the EQ-5D. In addition, it could potentially
be advantageous for decision-makers if the values of the bolt-on items could be related back to a standard tariff. However, based on the results presented here, a model with a simple decrement for each of the bolt-on levels is not appropriate. A more sophisticated analysis that takes into account both the severity of the bolt-on and the severity of the core EQ-5D state to which it is added may be feasible. Whether a full valuation of the EQ + bolt-on instrument is required for each new bolt-on item is not clear. Unfortunately,
the analysis comparing the coefficients of the models with and without the bolt-on was not conclusive.
It showed that there were no statistically significant differences between the coefficients at the 5% level.
However, the size of some differences in coefficients was not trivial and the lack of significant differences
could have been due to the sample size. There is also the possibility that the impact is specific to the
condition to which the bolt-on relates.
The limitations of the study include that the interviews were based in a specific region of the UK and may not
be generalisable to other countries or indeed regions in the UK, although there is no clear reason to suppose that the pattern of results would be different elsewhere. Some differences in reported problems with vision were found between the groups in the exploratory study; however, the regression analysis showed that these
characteristics did not significantly impact on values and the same finding was observed in the full valuation of
EQ + vision. Another limitation is the lack of qualitative research to investigate acceptability and alternative phrasing of the bolt-ons; however, the labelling builds on the framework of the EQ-5D and the qualitative research that has been used to develop it. Finally, this study has focused on the three-level version for the EQ-5D and it is not clear if similar results would be seen with thefive-level version.
A key feature of the EQ-5D is that it can be used across a range of conditions or diseases. This has a substantial advantage for economic evaluation and healthcare decision-making as it means decisions can
be based on a common measure and applied consistently across evaluations. For specific conditions, where
EQ-5D has been demonstrated to lack validity, the development of bolt-on instruments can offer a solution by improving the sensitivity of the instrument. While this may be at the expense of a level of consistency in the measurement and valuation of HRQL between conditions, retaining the EQ-5D as the basis for
measurement may be beneficial. By retaining the EQ-5D as the core basis for measurement and by using a
common valuation methodology, the degree of inconsistency in the estimates of HRQL is likely to be less
than if alternative GPBMs or condition-specific PBMs are used instead.
Conclusion
This report has presented three substantial pieces of research. We have considered when specific GPBMs
are appropriate for the measurement of HRQL, alternative methods for predicting outcomes when GPBMs have been found to be appropriate but data are unavailable and a method for developing bolt-ons to EQ-5D to improve its sensitivity. We have systematically reviewed the evidence on the performance
of EQ-5D and two other commonly used GPBMs in four, very broadly defined, clinical areas. We
was lacking for the latter. We also found that EQ-5D shows mixed results in vision impairment and
performs poorly in hearing-related conditions. Even where EQ-5D appears to be an appropriate measure of HRQL, data are not always collected within clinical studies. We have developed algorithms to predict
EQ-5D outcomes from two commonly used cancer-specific measures of QoL and explored a range of
alternative model specifications. Models predicting EQ-5D dimension-level responses performed best for
one of the measures (EORTC QLQ-C30); however, this approach did not work well in an alternative data set including the FACT-G as it included patients with a narrower range of disease severity. In this latter data set, when considering standard models, the OLS regression performed best in terms of the accuracy
of mean predictions for the whole sample and the subgroups defined according to severity. The LDVMM
outperformed the linear model in illustrative analysis of a selected model. Three bolt-on items to EQ-5D were developed and tested in an exploratory study and a bolt-on for vision was tested further and a full set of valuations for EQ + vision obtained. The results of these studies show that the inclusion of a bolt-on item has a complex impact on EQ-5D values and the results have important implications for that valuation of future bolt-ons.
Recommendations for further research
Generic preference-based measures are widely used in the economic evaluation of health interventions and are used to inform the decision-making of bodies such as NICE in the UK. The research presented here has consolidated some of the existing research in this area and presented new areas of methodology.
In order to ensure the most appropriate use of generic and condition-specific measures in HTA and
health-care decision-making, further research is required. We have highlighted the areas that we consider to be priorities for further research below.
Psychometric properties of the generic preference-based measures in different conditions
The reviews of the psychometric properties of the GPBMs focused on four broadly defined conditions: hearing
impairment, vision impairment, skin conditions and cancers. We recommend extending these reviews of the psychometric literature to more conditions. This would provide useful information and lead to recommendations on the use of the GPBMs for researchers conducting HTAs of interventions in other conditions.
Given the widespread use of the measures in HTA, the amount of evidence on psychometric properties of
the instruments was limited and, in most cases, the studies had not been specifically designed to examine
these issues. We recommend that more primary research or analyses of primary data sets into the
psychometric properties of GPBMs is undertaken, particularly in cancer, and particularly of the reliability of the measures in the other conditions.
Mapping
It was not possible to validate the mapping functions estimated in this project using an external data set, but this is recommended to assess the external validity of the functions.
In addition, we recommend comparing alternative statistical models in larger data sets, including those for EORTC QLQ-C30 and FACT-G.
The development and use of bolt-ons to EQ-5D
The development and use of bolt-ons to EQ-5D is still a new but growing area of methodological research. The research presented in this report offers insights that can be used when developing future bolt-ons. Further research to validate the EQ + vision measure presented here would be useful. The results of the
exploratory study of the hearing and tiredness bolt-ons suggest that these measures would also benefit
from validation and further valuation. There are still methodological issues relating to bolt-on development that require further investigation. We recommend that the best way to undertake this is to develop a systematic programme of research into bolt-ons for EQ-5D.