The EQ-5D is a well-established and widely-used generic instrument for assessing health-related quality of life (HRQL). It is a two-part questionnaire, designed for self-completion. In the first part, the respondent describes his or her prevailing state of health in terms of perceived problem severities for five domains. These domains are limitations on mobility, capacity for self-care, ability to conduct usual activities, pain & discomfort and anxiety & depression, in that order. Three severity descriptions are available within each domain in the traditional version of the instrument (EQ-5D-3L), namely, none, moderate and severe/extreme (coded 1 through 3, respectively). This descriptive system accommodates 243 possible health states, each defined as a different vector or profile, ranging from 11111 (no problems in any domain) to 33333 (severe problems in all domains). In the second part of the instrument, the respondent evaluates his or her prevailing state of health by indicating a position on a visual analogue scale (VAS). This is a vertical, calibrated, line, anchored at 0, the “worst health state imaginable ”, and at 100, the “best health state imaginable ” .
The principal use of the EQ-5D instrument to date has been in health technology assessment (HTA); indeed, the UK’s influential National Institute for Health and Clinical Excellence has declared the EQ-5D to be “the preferred measure of HRQL in adults”  p. 38. For the purposes of cost utility HTA, the first (descriptive) part of the questionnaire is of far greater significance than the second (evaluative) part. This is because HTA conventions in most countries  dictate that, whilst the health effects of interventions during clinical trials should be assessed using the health state descriptions made by the participating individuals, the values of those states should derive from public or social, rather than individual, judgements . These social health state utilities are obtained from independent studies which aggregate the opinions of members of general populations into state-specific “index scores”, anchored at 1 (EQ-5D state 11111) and 0 (dead).
As EQ-5D VAS results have had little relevance in HTA, the relationship between individuals’ descriptive profiles and their corresponding VAS scores has not been extensively researched. Only a few regression studies have been undertaken, predicting VAS scores from reported problem severities by domain, coded as binary dummy variables. These studies confirm the intuition that subjects who report more health problems at greater severities tend to indicate poorer HRQL in the form of lower VAS scores. For example, a UK analysis of a large sample of pooled records for surgical procedures reported that “the binary variable coefficients are all in the expected direction and are highly statistically significant. Moreover, they are consistent in each dimension, so that the coefficients on level 3 are all higher than the coefficients of level 2. The differences between the level 2 and level 3 scores are all significant”  p. 16-17. Such analyses also suggest that problem severity variations in the five domains are, of themselves, insufficient to explain VAS scores fully [6–8].
A new role for the EQ-5D raises further concern over the absence of investigations of the association between profiles and VAS scores. Since 2009, the National Health Service (NHS) in England has been collating and publishing data from a suite of “before and after” patient-reported outcome measures (PROMs) . PROMs are intended to facilitate comparisons of provider performance and of patient benefit from services. Although the PROMs include both components of the EQ-5D, the performance of the VAS was excluded from appraisal of the pre-implementation pilot study . Presently limited to four types of elective surgery (hip, knee, hernia repair and varicose veins), there exists an explicit intention to “extend PROMs across the NHS wherever practicable”  p. 14. Assuming that such an extension were to occur, published VAS and index scores would become available and might be used for comparison across different conditions, despite the validity of any such comparison remaining un-established. The potential problem is not confined to England, as other countries, including Sweden  and Canada , have indicated an interest in publishing EQ-5D results as part of their own PROMs packages.
The EQ-5D aims to be a “non-disease-specific instrument for describing and valuing health-related quality of life”  p. 337, and it was the promise of generality which made the instrument attractive as a PROM . For this aspiration to be realised, however, it is necessary to suppose that the association between EQ-5D profile and VAS score does not vary systematically by medical condition or circumstances. Were the converse to be the case, it would follow that VAS results are not necessarily comparable across conditions or interventions, even when subjects are ostensibly in the same health states. In this paper, we consider the supposition as a hypothesis and investigate the association between VAS scores and health states amongst subjects experiencing different interventions.