Abstract

Background

When the EuroQol EQ-5D is applied in settings other than resource allocation, a non–preference-based score may be more appropriate than societal, preference-weighted utility. To develop a psychometric score for the EQ-5D, its structural relationship, ie, how the 5 items/dimensions interrelate, must be understood to inform appropriate methods of summarizing the instrument.

Objectives

To explore psychometrically derived approaches of elucidating the 5-level EQ-5D (EQ-5D-5L) item structure.

Methods

Three measurement models were assessed. All 5 items were modeled as reflective indicators using confirmatory factor analysis. EQ-5D-5L items were conceptualized as formative indicators, and other health scales (eg, the short form 36 health survey) were conceptualized as reflective indicators in Multiple Indicators Multiple Causes models (external MIMIC). The EQ-5D-5L items were modeled as a combination of formative and reflective indicators in internal MIMIC models. Results across 9 data sets from various countries and patient groups were examined to determine their robustness.

Results

All items loaded well (0.63-0.96) in the confirmatory factor analysis except for anxiety/depression (0.20-0.66, excluding 1 outlier). The model fit statistics of the external MIMIC models were poor, and the coefficients of the Self-Care dimension were small. The internal MIMIC model with Mobility, Pain/Discomfort, and Anxiety/Depression as formative indicators and Self-Care and Usual Activities as reflective indicators fit best. The model results of the Spanish valuation data set were outliers.

Conclusions

Although there were some variations in results across subgroups, the relationship between the items remained robust. The evidence calls for testing of formative/reflective combination approaches to summarize the EQ-5D-5L.