Cross-cultural measurement equivalence of the 5-level EQ-5D (EQ-5D-5L) in patients with type 2 diabetes mellitus in Singapore

In outcomes research, measurement equivalence is achieved if a scale generates comparable scores for individuals at the same level of health regardless of the populations they come from [1]. It is important to test the cross-cultural measurement equivalence of a self-reported health-status scale that is intended to compare health outcomes of populations from different cultures. This is because individuals from different cultures may have different ways of living, thinking, and expressing [2], leading to culture-specific interpretation of questionnaire items and/or response styles and difference in scale scores. When such difference is large enough, measurement equivalence cannot be assumed.

The 5-level EQ-5D (EQ-5D-5L) is a new version of the EQ-5D, a brief, generic health-status instrument [3]. It has been shown to have good psychometric properties [4‐6] and suffer from fewer ceiling effects than the original version, i.e., the 3-level EQ-5D (EQ-5D-3L) [4‐7]. The first part of the instrument contains 5 five-point Likert-type items (no/slight/moderate/severe/extreme), which describe five dimensions of a respondent’s health status on the day of the survey, i.e., mobility, self-care, usual activities, pain/discomfort, and anxiety/depression. An individual’s responses to the five items jointly form a multi-attribute health state for which a utility value (i.e., the EQ-5D-5L index score) can be assigned to indicate the utility of the health state to the general public [8]. The index score is anchored by 0 (death) and 1 (full health), with a higher score indicating higher utility. The second part is the EQ-visual analog scale (EQ-VAS), which is a vertical, 0 (the worst health state) to 100 (the best health state) hash-marked numerical rating scale, to rate respondents’ overall health.

The EQ-5D-5L questionnaire has been available in the official languages of Singapore, a multicultural, multiethnic city-state in South East Asia. Measurement equivalence is important to the use of health-related quality of life (HRQoL) instruments in Singapore because none of the official languages is spoken fluently by all residents, although many of them are multilingual. However, measurement equivalence of the EQ-5D-5L across different language subgroups of the Singaporean population is unknown. Therefore, this study aimed to assess the measurement equivalence of the EQ-5D-5L index score (calculated using an interim algorithm) and the EQ-VAS score among English, Chinese, and Malay versions in patients with type 2 diabetes mellitus (T2DM).

Seven hundred and twenty-nine patients participated in the study, representing an overall response rate of 61.5 %. Participants completing different language versions differed in some characteristics (Table 1). More severe levels of problems were less endorsed by all language groups for all health dimensions of the EQ-5D-5L (Table 2). Participants completing the Malay version (mean ± standard deviation [SD]: 81.85 ± 15.04) had significantly higher mean EQ-VAS score than those who completed the English (mean ± SD: 75.46 ± 18.46) or Chinese (mean ± SD: 78.00 ± 18.33) version (p < 0.001). No statistically significant difference was found in the trend of responses and the mean EQ-5D-5L index score across language.

Adjusted and unadjusted results from the linear regression analyses are shown in Table 3. After adjusting for the covariates, the mean EQ-5D-5L index score of the Chinese version was higher than that of the English version; the Malay version had a lower mean EQ-5D-5L index score than the English and Chinese versions. Comparisons of the 90 % CIs of the differences with the respective pre-determined equivalence margin suggested that, equivalence of the EQ-5D-5L index scores was demonstrated between the Chinese and English versions and between the Malay and English versions, whereas equivalence could not be determined between the Malay and Chinese versions. The adjusted mean EQ-VAS scores of the Chinese and Malay versions were higher than that of the English version. The Malay version had a higher adjusted mean EQ-VAS score than the Chinese version. The 90 % CIs of the differences suggested that, while equivalence of the EQ-VAS scores was demonstrated between the Chinese and English versions and between the Malay and Chinese versions, equivalence could not be determined between the Malay and English versions.

Adjusted and unadjusted results from the logistic regression analyses are presented in Table 4. After adjusting for the covariates, participants completing the Malay version were less likely to report ‘no problems’ in mobility than those completing the Chinese version (adjusted odds ratio: 0.435; 95 % CI: 0.221 to 0.855). Other between-language differences were not statistically significant.

Assessment of self-reported health outcomes in Singapore usually involves multiple ethnic groups, which necessitates the use of more than one survey language. Therefore, only cross-culturally equivalent instruments would provide the most valid measurement in such a setting. In this study, measurement equivalence was found between the Chinese and English versions and between the Malay and English versions of the EQ-5D-5L index scores. Measurement equivalence was also found between the English and Chinese versions and between the Malay and Chinese versions of the EQ-VAS scores. The findings are consistent with a previous study, which reported that the 90 % CI of difference in EQ-5D-5L index and EQ-VAS scores between the Chinese and English versions were −0.02 to 0.06 and −5.30 to 5.50, respectively [21]. However, it should be noted that the Chinese and English EQ-5D-5L questionnaires used in that study were not official versions, although the study participants were Singaporean residents.

Nevertheless, equivalence of the EQ-5D-5L index scores could not be determined between the Malay and Chinese versions. Participants using the Chinese version reported better overall health status. This is consistent with previous studies, which found that ethnic Chinese were more likely to endure health problems than other ethnicities [22, 23]. Indeed, our analyses of the participants’ response patterns to the EQ-5D-5L items suggested that participants using the Chinese version, who were all ethnic Chinese, were less likely to report mobility problems than those using the Malay version, who were mainly ethnic Malay and Indian. Equivalence of the EQ-VAS scores between the Malay and English versions could not be confirmed. Patients completing the Malay version had higher adjusted mean EQ-VAS score than those completing the English version, indicating that the former would give higher rates to their overall health than the latter even if they were in same level of health. One explanation could be that the EQ-VAS has been found to be a more mental than physical health measure [24]; studies conducted in Singapore consistently found that Malays reported better mental health than other ethnicities [23, 25].

It should be noted that a comprehensive assessment of the cross-cultural measurement equivalence of the EQ-5D-5L should also include responses to the five items. The individual EQ-5D-5L items have also been used as independent outcome measures, and their cross-cultural equivalence cannot be inferred from that of the index or VAS score. Assessing the equivalence of the items, however, would require a sample size larger than the one we used in the current study. Therefore, we did not perform the equivalence analysis for the EQ-5D-5L items; it would not be informative to conclude that the equivalence of the items between any two language versions cannot be determined. The cross-cultural equivalence of the EQ-5D-5L at the item level should be examined in the future when suitable datasets are available.

This study has a few limitations. First, the convenience sample used in this study may have led to selection bias, as patients who had poorer health may have been less willing to participate in the survey. Second, the EQ-5D-5L index score was calculated using an interim algorithm, mapped to the general UK population-based EQ-5D-3L value set, which may not fully reflect the measurement properties of the index score obtained from direct valuation of the EQ-5D-5L health states. Third, most clinical data (e.g., presence of T2DM-related complications and comorbidities) used in this study were patient-reported, which may not be accurate.

In conclusion, this study provides evidence for the measurement equivalence of the EQ-5D-5L instruments across language, in a multicultural, multiethnic Asian population with T2DM. Future studies are needed to investigate the cross-cultural measure equivalence of the EQ-5D-5L items and whether this research finding can be generalized to other populations.