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We read with interest the recent article by Schraget al,1 in which the authors showed that the EuroQol-5D (EQ-5D) is a valid measure of quality of life in patients with Parkinson's disease. However, the authors neglected to mention two important aspects of the EQ-5D that differentiate it from typical quality of life instruments.
Firstly, the EQ-5D is a preference based measure. The summary score of the EQ-5D captures the strength of a person's preference for a given health outcome relative to other possible outcomes. These preferences (also called utilities) are measured on a scale from 0 to 1 where 0 represents death and 1 represents perfect health. The values derived from the EQ-5D can be used to compare health states in a quantitative way. For example, a health state with a value of 0.5 is half as desirable as perfect health. The scoring rule for the EQ-5D permits scores less than 0, implying that some health states may be worse than death.
Secondly, the index scores for the EQ-5D are intended to approximate general population preferences rather than the respondent's own health values. The EQ-5D values were developed based on ratings by a large, random sample of the adult population of the United Kingdom, The Netherlands, and Sweden.2 As a result, the summary index score does not quantify the respondent's value of their own health, but rather the value that the general population would place on the respondent's health. By contrast, the visual analogue scores are direct measures of the value a respondent places on his or her own health. It is not surprising that the summary index scores and visual analogue scale scores are somewhat different in the study by Schraget al.
These properties of the EQ-5D make it an indicated measure of health status for certain applications, particularly estimating health utility for cost-utility analysis. It is, perhaps, a fortunate accident that it is also a valid measure of quality of life in patients with Parkinson's disease.
Selai et al reply:
We thank Siderowf and Werner for their interest in the EQ-5D and our work. They raise important points about the use and interpretation of generic quality of life instruments.
The valuation of health states raises many complex methodological and ethical issues and it is the topic of considerable debate in the literature.1-1 Although we have participated in this debate,1-2 we did not enter into a discussion of these issues in our recent paper because this was beyond the scope of that study.
The EQ-5D is a generic measure that has three distinct components, each providing separate data. The first part yields a simple descriptive profile of the respondents' own subjective health status in five dimensions. Secondly, the respondents next rate their own health on a visual analogue scale, calibrated 0–100. Thirdly, according to how the respondents have rated themselves on the descriptive profile, a utility value can be ascertained. Thus, the EQ-5D generates a cardinal index of health, giving it considerable potential for use in both economic evaluation and for ascertaining a person's subjective perspective of their own health status. The EQ-5D classification system defines a fixed number of health states, which may include health states valued worse than death, but leaves open the issue of what value should be assigned to each of those states.1-3 Valuation data sets have been obtained in several countries, both European and non-European.
The evaluation of health related quality of life (HR-QOL) of patients with a given disease is generally measured using a disease specific instrument,—for example, the PDQ-39 in patients with Parkinson's disease. As these instruments are only applicable to patients with a particular disease, they do not allow comparisons across health conditions and are of limited use in economic studies. It is therefore recommended that a generic HR-QOL instrument be used in addition to disease specific measures. The generic measure must be tested and validated before use in the respective patient population. The purpose of our study was to test the validity and feasibility of two generic measures of HR-QOL (the EQ-5D and the SF-36) in patients with Parkinson's disease. The EQ-5D, a simple generic instrument, was shown to have good validity and feasibility and performed better than the SF-36 in this group of patients with Parkinson's disease.