A comparison of generic, indirect utility measures (the HUI2, HUI3, SF-6D, and the EQ-5D) and disease-specific instruments (the RAQoL and the HAQ) in rheumatoid arthritis

Abstract

Rheumatoid arthritis (RA) is a common, chronic disease where health-related quality of life (HRQL) is one of the main goals of therapy. As such, instruments used to measure HRQL in RA must be able to discriminate across RA severity. The two basic categories of instruments used to measure HRQL are generic instruments and disease-specific instruments. Generic instruments can be further subdivided into preference-based measures which yield both single and multi-attribute utility values anchored at zero (death) and 1.00 (perfect health) as a measure of HRQL. The scores from these types of instruments can be integrated into cost-utility analyses as the weightings for quality adjusted life years. We assessed the construct validity of utility scores from four generic preference-based measures (the Health Utilities Index 2 and 3 (HUI2, HUI3), the EuroQol 5D (EQ-5D), and the Short Form 6-D (SF-6D) and disease specific measures (the Rheumatoid Arthritis Quality of Life Questionnaire (RAQoL) and the Health Assessment Questionnaire (HAQ)) in a sample of 313 RA patients in British Columbia, Canada. We also estimated the minimally important differences (MID) for each of the measures. Generally, as anticipated, the disease-specific measures were better able to discriminate across groups with higher RA severity; however, utility scores from each of the scales also appeared to discriminate well across RA severity categories. The MID values agreed with those previously reported in the literature for the HUI2, SF-6D and the HAQ and provided new information for the HUI3, EQ-5D and the RAQoL. We conclude that the all of the preference-based utility measures that were evaluated appear to adequately discriminate across levels of RA severity.

Introduction

Rheumatoid arthritis (RA) is a chronic, progressive disease that places a substantial burden on those afflicted and their families. Specifically, RA, its treatments, and complications arising from both, result in detrimental effects on many areas of life including physical, psychological, and social functioning (Tijhuis et al., 2001). Yet many clinical measures do not adequately capture the overall impact of the disease on individuals. Furthermore, because one of the goals for therapeutic interventions in RA is to improve health-related quality of life (HRQL) (American College of Rheumatology Subcommittee on Rheumatoid Arthritis Guidelines, 2002), it is important to utilize instruments that display appropriate measurement properties.

The two basic categories of instruments used to measure HRQL are generic instruments and disease-specific instruments (Guyatt, Feeny, & Patrick, 1993). Utility, or preference-based, measures are an example of generic instruments that are derived from decision and utility theories. As such, preference-based approaches integrate different aspects of health into a single index anchored by a value of ‘1.00’ for full health and ‘0’ for death (health states considered worse than death can be represented by negative values). In turn, these measures are used in economic evaluations to integrate survival and HRQL into a single metric, the quality adjusted life year (QALY) (Drummond, O’Brien, Stoddart, & Torrance, 1997).

Health utilities can be either measured directly (using techniques such as the standard gamble or time trade-off) or indirectly (using multidimensional HRQL questionnaires developed using multi-attribute utility theory (MAUT)) such as the Health Utilities Index 2 and 3 (HUI2 and HUI3) (Torrance et al., 1996; Feeny et al., 2002) the Short Form 6D (SF-6D) (Brazier, Roberts, & Deverill, 2002) and the EuroQol (EQ-5D) (The EuroQol Group, 1990). Due to their ease of administration, these indirect measures are commonly used as the source of quality weightings in economic evaluations. A brief overview of these instruments has been provided in Table 1 and comprehensive reviews are available. (Drummond et al., 1997; Kopec & Willison, 2002).

Disease-specific measures are commonly utilized to assess HRQL in RA (Lubeck, 2002). Specifically, the Health Assessment Questionnaire (HAQ) Disability Index (Bruce & Fries, 2003) is a commonly used disease-specific measure whereas the Rheumatoid Arthritis Quality of Life (RAQoL) questionnaire (de Jong, van der Heijde, Mckenna, & Whalley, 1997) is a relatively newly developed instrument. The HAQ was originally developed as one of the first self-report, functional status (disability) measures and has become one of the dominant instruments in musculoskeletal diseases including RA (Bruce & Fries, 2003). The HAQ has been utilized to assess disability for approximately two decades and is often an outcome for clinical trials in RA. The RAQoL is the first patient-completed instrument specifically designed for use with RA patients (de Jong et al., 1997).

Compared to generic measures of HRQL, disease-specific measures focus on the particular problems that are often unique to the disease that they are developed to assess. As such, these measures may have greater ability to measure functional impairments resulting from the disease and detect smaller changes in health relative to generic measures. However, generic measures permit comparisons across disease states, which may provide useful data for health policy and resource allocation decision-making.

Furthermore, any instrument utilized to assess HRQL needs to be valid and reliable (Streiner & Norman, 1995). Generally, validity can be defined as the extent to which an instrument measures the property that it is intended to measure (Streiner & Norman, 1995). Construct validity is an assessment of the extent to which the scores of an instrument correlate with other hypothesized measures or indicators of the health concept or concepts of interest. (Streiner & Norman, 1995; Brazier & Deverill, 1999).

In a recent paper by Brazier and Deverill (1999) concern is voiced that the aspect of health used to subdivide groups in assessing construct validity might not reflect preferences (for example, age). However, they stipulate that the researcher may carefully hypothesize or construct the expected difference based on preferences. Thus, as long as care is taken in selecting the groups such that preferences for health states would be expected to differ, this would be an appropriate method to assess the construct validity of preference-based instruments.

There remains a gap in the literature regarding the assessment of construct validity for indirect measures of health utility. Although construct validity has been investigated for the HUI2 and HUI3 in Type 2 diabetes (Maddigan, Feeny, & Johnson, 2004) the HUI3 in self-reported stroke and arthritis (Grootendorst, Feeny, & Furlong, 2000), the HUI2 and HUI3 for Alzheimer's disease (Neumann et al., 2000), the HUI3 and the EQ-5D in intermittent claudication (Bosch & Hunink, 2000) and the EQ-5D in RA (Hurst, Kind, Ruta, Hunter, & Stubbings, 1997), there are little comparative data across all four instruments in the same patient sample with a well, delineated, chronic disease. In addition, there are no data that compare the construct validity of disease-specific instruments like the HAQ and the RAQoL Questionnaire. Thus, the objectives of this study were to examine the cross-sectional construct validity of the global and single-attribute scores from the indirect utility instruments in terms of their ability to distinguish between subgroups of individuals with different levels of RA severity, compare amongst them and compare them to disease-specific instruments. In addition, for each of the instruments, the minimally important difference was determined and was compared to previously defined values where available.