OBJECTIVES: The EQ-5D-5L is worded to assess domains of health using single items. Other measures of health assess domains using multiple item dimensions. For example, EQ-5D-5L measures mobility using one item, whereas SF-36 and PROMIS-29 measure physical functioning using ten and four respectively. This raises the question of the measurement sensitivity of the different approaches. The aim of this study was to investigate the measurement characteristics of the EQ-5D mobility dimension, and the SF-36 and PROMIS-29 items measuring physical functioning, using item response theory (IRT).

METHODS: Data from 794 Australians with a number of common health conditions (arthritis, depression, pain and diabetes) who completed all three instruments were used. Exploratory factor analysis (EFA) was used to examine whether there was a single domain of mobility/physical functioning being measured across the instruments. Secondly, graded response IRT was conducted on the resulting domain to understand the sensitivity and measurement coverage of each item across the underlying severity continuum, and the measurement characteristics of the domain.

RESULTS: EFA demonstrated that the EQ-5D-5L mobility dimension, the ten SF-36v2 and four PROMIS-29 items measuring physical functioning loaded as a single domain. The IRT analysis found that different information profiles across the items. The EQ-5D-5L mobility dimension was found to have measurement sensitivity to the moderate range of the underlying continuum measuring mobility problems. The SF-36v2 and PROMIS-29 items were generally sensitive to smaller range of the underlying severity continuum. However, the items were sensitive at different points of the underlying physical functioning severity scale.

CONCLUSIONS: The item wording used for EQ-5D-5L mobility is advantageous in assessing general problems, but more specifically worded items allow for a more focused assessment of problems across different levels of physical severity. These results provide a detailed understanding of the measurement of physical functioning using a novel IRT approach.