A Differential Item Functioning Analysis of the EQ-5D in Cancer.

OBJECTIVES: To determine whether differential item functioning (DIF) was present in the EuroQol five-dimensional questionnaire (EQ-5D) used in cancer (non-small cell lung cancer and prostate cancer).

METHODS: The Partial Credit Model was applied to the three-level version of the EQ-5D with data obtained from four randomized controlled trials in prostate cancer and non-small cell lung cancer completed at baseline before treatment (N = 2213). DIF was assessed across cancer type (two levels), sex (two levels), and age group (three levels) using Mantel-Haenszel chi-square statistics and evaluated against the Educational Testing Service classification rules.

RESULTS: The presence of DIF was determined in 14 of 25 (56%) potential DIF contrasts in all the EQ-5D domains. Although mostly the DIF was categorized as either negligible (3 of 25 [12%]) or medium (7 of 25 [28%]), large DIF was observed in 4 of the 25 contrasts (16%). The mobility domain, in particular, showed consistently large DIF across cancer type, sex, and age.

CONCLUSIONS: Given the use of the instrument in health status assessments across conditions and interventions, these results may have significant implications for the EQ-5D in health economic evaluations. Further research is warranted to determine whether these results hold for other cancers.