Do English and Chinese EQ-5D versions demonstrate measurement equivalence? an exploratory study

A consecutive sample of outpatients with rheumatic diseases seen at a tertiary referral hospital within a 2-week period were interviewed by trained nurse interviewers using an identical English or Chinese questionnaire containing the Singaporean English or Chinese EQ-5D, a 10 cm pain visual analog scale (VAS), and assessing psychosocial, socio-demographic and other variables. Written consent was obtained from each subject for this IRB-approved study. Inclusion criteria were physician diagnosis of a rheumatic disease and ability to cooperate with the interview. This research was part of a larger study of English and Chinese EQ-5D versions in subjects with rheumatic diseases [10, 11].

The EQ-5D consists of a health descriptive system and a visual analog scale (EQ-VAS) for respondents to self-classify and rate their health on the day of administration of the instrument [6, 7]. The descriptive system has 5 items/dimensions (i.e., mobility, self-care, usual activities, pain/discomfort and anxiety/depression), and for each item, there are three response levels (i.e., no problems, moderate problems and extreme problems). The items can be used individually or in combination (as a health profile) as descriptive measures in clinical studies. Theoretically, the design of the descriptive system identifies as many as 243 unique health states, although a small number of these health states are not plausible in reality [12]. Scoring methods have been developed to assign each of these health states a utility score in which 1 represents full health (no problems with all 5 items) and 0 represents being dead [12, 13]. EQ-5D utility scores can be used to calculate quality-adjusted life-years (QALYs) for cost-utility analysis of health interventions [14, 15]. The EQ-VAS is a vertical, graduated (0–100 points) 20 cm 'thermometer', with 100 at the top representing 'best imaginable health state' and 0 at the bottom representing 'worst imaginable health state'. The EQ-VAS score can be used as a measure of clinical outcome, using individual respondents' own judgment [16].

Singaporean English and Chinese EQ-5D versions were derived by adapting the UK English [7] and the Taiwan Chinese versions respectively, using the EuroQol Group's guidelines for cultural adaptation of the EQ-5D [8]. The resulting Singaporean English EQ-5D is identical to its source UK English version except that the word 'box' in the instructions for the EQ-VAS was replaced with 'BLACK BOX' [10]. We found this amendment improved respondents' compliance with EQ-VAS instructions to link the box representing 'your own health state today' to the scale. Changing the word 'box' to 'BLACK BOX' was also adopted for the Singaporean Chinese EQ-5D [11].

We evaluated the equivalence of English and Chinese EQ-5D versions by examining whether score differences between these versions were clinically important. Based on the definition of Drasgow and Kanfer [2], if these differences were clinically unimportant, these versions would demonstrate measurement equivalence. Using methodology for assessing therapeutic equivalence in clinical trials [17, 18], we therefore compared the 95% confidence interval (95%CI) of EQ-5D item, utility and VAS score differences with pre-defined equivalence margins to determine if differences in scores were clinically important or unimportant. Each equivalence margin represented a range of score differences which would be too small to be clinically important. Comparing the 95% CI for a score difference with its corresponding equivalence margin could lead to 1 of 3 possible results [17, 18], illustrated graphically in Figure 1. First, if the 95%CI fell completely within the equivalence margin, the score difference would be clinically unimportant, and measurement equivalence would be demonstrated (Figure 1, option A). Second, if the 95%CI did not overlap with the equivalence margin at all, the score difference would be clinically important, and non-equivalence would be demonstrated (Figure 1, options E and F). Third, if the 95%CI partially overlapped with the equivalence margin, the score difference might or might not be clinically important, and either equivalence or non-equivalence might be present (Figure 1, options B, C and D).

We defined equivalence margins for responses to items, utility and EQ-VAS scores after calculating utility scores using the algorithm developed by Dolan [12]. Ideally, each equivalence margin should be based on the minimal clinically important difference (MCID) [19] for that score. However, as these have not been specified in the EQ-5D literature, we defined equivalence margins based on the best available data (see below). Score differences were assessed using logistic regression models for responses to EQ-5D items and linear regression models for EQ-5D utility and EQ-VAS scores, with or without adjustment for influence of other variables which might influence HRQoL (e.g. age, gender, socio-economic status). It was important to adjust for the influence of these variables when assessing influence of language, as observed differences in EQ-5D scores might be caused by these determinants rather than by questionnaire language.

Score differences for each EQ-5D item were examined using logistic regression models (one model for each item). Responses to each item were treated as a binary dependent variable (no problems = 0/with problems = 1) by combining response levels 2 (moderate problems) and 3 (extreme problems). Language version was coded into a dummy variable (Chinese = 0/English = 1). Each model was constructed by first entering language version as the only independent variable; other selected independent variables were entered subsequently. These variables were age, gender, years of education, employment status and pain VAS score, which were selected because they differed substantially between the two groups (Table 1) and have been reported to correlate with responses to the EQ-5D [20‐26]. Age and pain VAS scores were treated as continuous variables while the remaining variables were coded into dummy variables. In the model for pain/discomfort item, pain VAS score was not included, as both variables measure pain and therefore did not represent an independent-dependent relationship (a necessary assumption for regression models [27]). Based on previous study designs for detecting therapeutic equivalence [18, 28], we pre-defined an equivalence margin of (-10%, +10%) for each EQ-5D item. Thus, if the difference in the proportion of subjects reporting problems for an item was less than ± 10% between the 2 language groups, the item demonstrated measurement equivalence as defined for this study. The 95%CI of the regression coefficient (i.e. odds ratio) for language version was converted into the 95% CI of the proportion derived from this odds ratio to facilitate comparison with the equivalence margin of (-10%, +10%) [29].

Score differences in EQ-5D utility and EQ-VAS scores were examined using separate linear regression models, with and without adjustment for variables potentially influencing HRQoL (listed above). The 95%CI of the regression coefficient of language version was compared with pre-defined equivalence margins to determine measurement equivalence. We reviewed the literature but found no reports regarding the MCID of EQ-5D utility or EQ-VAS scores; however, we found that differences in mean EQ-5D utility and EQ-VAS scores in individuals differing in health status were seldom less than 5% of their entire score ranges (i.e., 0.05 points for utility scores and 5.0 points for EQ-VAS scores) [23‐25, 30‐33]. We therefore defined equivalence as a difference of less than 5% of the respective score ranges, that is, (-0.05, +0.05) for utility and (-5.0, +5.0) for EQ-VAS scores.

The mean (SD) age of the 114 interviewed subjects was 49.4 (16.5) years, with 81.6% (n = 93) being female and 57.9% (n = 66) completing the English EQ-5D. English EQ-5D respondents were younger (44.3 vs. 56.7 years, p < 0.001) and more likely to be male (27.3% vs. 6.2%, p < 0.01), employed (57.5% vs. 27.1%, p = 0.01), better educated (6 or less years of education: 12.1% vs. 64.6%, p < 0.001) and reported less pain (mean pain VAS score: 3.3 vs. 4.7, p < 0.01) than Chinese-speaking respondents. Other characteristics of the two groups of subjects are summarized in Table 1.

Equivalence was assessed for mobility, usual activities, pain/discomfort items and anxiety/depression items, but not for the self-care item as subjects completing the English EQ-5D reported no problems with this item (Table 1), which did not allow a logistic regression model to be created. The 95%CIs of odds ratios and corresponding proportion intervals for language version are summarized in Table 2. Before adjusting for other variables, the influence of language partially overlapped with the equivalence margin of (-10%, +10%) for all items, corresponding to options B or D in Figure 1. The lower bound of the 95%CIs ranged from -20.9 to -17.0%. The upper bound of the 95%CI for mobility, usual activities and pain/discomfort items was less than or approximately +10%; that for the anxiety/depression item was 17.1%. After adjusting for the influence of other variables, the 95%CIs changed slightly but still overlapped partially with the equivalence margin for all items. The lower bound ranged from -29.9 to -20.0%; the upper bound was less than or approximately +10% for 3 items and 26.3% for the anxiety/depression item.

The 95%CIs of effect sizes for language version on EQ-5D utility and VAS scores are summarized in Table 3. Before adjusting for other variables, the influence of language partially overlapped with the equivalence margin for both utility and VAS scores, with the 95%CI of the effect of language version being (-0.07, +0.09) and (-3.5, +8.8), respectively. This corresponded to options D and C respectively in Figure 1. After adjusting for the influence of other variables, the 95%CIs were (-0.14, +0.03) and (-11.6, +3.3) respectively, which again partially overlapped with the respective pre-defined equivalence margins of (-0.05, +0.05) and (-5.0, +5.0).