Dementia is a devastating condition for patients and caregivers and a major public health concern due to its increasing incidence. Assessment of meaningful treatment benefits is complex. Many researchers state that cognitive response no longer suffices in anti-dementia trials. There is emerging consensus on the value of patient-reported outcomes such as health-related quality of life (HRQoL). There are two fundamentally different approaches to measuring HRQoL. The first is the standard ‘questionnaire’ approach, using descriptive or profile instruments. The second is the ‘index’ approach, using preference-based instruments[4, 5].
Descriptive instruments summarize multiple domains of health status and are based on classical test theory. A small set of related items covers the content of various health domains and a score for each dimension is generated. One such frequently used generic descriptive instrument is the SF-36. Examples of descriptive instruments that are used in dementia include the Quality of Life in Alzheimer’s Disease (QOL-AD) and the Dementia Quality of Life Instrument (D-QOL)[8, 9].
Index measures quantify multiple health domains into one single metric figure. In the case of HRQoL, index measures quantify the desirability of a certain health state. The generated values, variously called utilities, preferences or weights, are often unambiguous; e.g., a value of 1.0 stands for ‘perfect health’, 0.0 for ‘death’. HRQoL values with metric characteristics are especially useful because they are applicable in health outcome research and economic evaluations. Descriptive tools lack this feature. The EuroQol-5 D (EQ-5D) is the most widely used generic HRQoL index instrument[11, 12]. It includes the five dimensions mobility, self-care, usual activities, pain/discomfort and anxiety/depression.
Both descriptive and index instruments have generic and disease-specific versions, based on the extent to which illnesses are covered. Disease-specific instruments target individual diseases or specific health problems, while generic instruments are more universal and cover general health aspects.
Recently, Riepe et al. concluded that current HRQoL-index instruments, which have been useful in other contexts, are ill-suited and insufficiently validated to play a major role in dementia research, decision making and resource allocation. They reported that six cost-effectiveness studies, using quality-adjusted life years (QALY) measurements, were unsatisfactory, and that large gaps existed between published measurements of HRQoL and the quality standards required by guidelines. Their conclusion was supported by the consensus statement of the International Psychogeriatric Association that generic HRQoL index measures, such as the EQ-5D, are not satisfactorily validated in dementia and that this called into question previous health economic analyses. The solution seems to be a disease-specific HRQoL index instrument. Such instruments have been developed for various diseases but not for dementia[15–19]. We therefore designed a dementia-specific index instrument, the Dementia Quality of life Instrument (DQI).
The DQI is a classification system based on the conceptual framework of the EQ-5D. We replaced the generic EQ-5D domains by domains that are better able to describe the health status in dementia. Our paper presents evidence for the construct validity of the DQI by a detailed listing of the steps taken to prove that the chosen domains indeed represent the construct. Additionally, we undertook a survey under dementia professionals on the contents of the instrument. Next, relations to other variables were examined in dementia patients and their informal caregivers by correlating DQI scores with scores from two well-validated quality of life instruments, one generic and one dementia-specific. Finally, we report on the feasibility of the DQI in dementia patients and caregivers.