OBJECTIVES: To explore the item performance of the EQ-HWB using Item Response Theory (IRT), with a focus on including respondents with limited cognitive capacity.

METHODS: We conducted face-to-face interviews to recruit over 500 respondents with limited cognitive capacity. Initially, we used the criteria of > 70% ceiling effect per item and 5% missing data to determine potentionally problematic response in this population. We then randomly selected 150 respondents for Exploratory Factor Analysis (EFA) with a Promax oblique rotation and used the remaining respondents for Confirmatory Factor Analysis (CFA). Finally, we applied IRT to observe item performance and tested for Differential Item Functioning (DIF) based on age and gender variables.

RESULTS: We recruited 553 respondents: 96 patients with lung cancer, 125 with diabetes, 102 with depression, 113 with schizophrenia, and 101 healthy individuals. Items 'wash', 'outside', 'inside', 'doing activities', and 'hearing' exhibited a ceiling effect, and no items had missing values greater than 5%. Three oblique factors explained the factor structure of the EQ-HWB: 'psychological well-being', 'physical functioning', and 'physical sensation'. The CFA bi-factor model showed good fit indices with CFI: 0.976, TLI: 0.97, and RMSEA: 0.059. Items 'cope', 'look forward', 'inside', 'outside', and 'wash' had unfavorable Item Characteristic Curves (ICCs). Items 'accepted', 'cope', 'remembering', 'outside', and 'exhausted' exhibited age-related DIF. Two items, 'accepted' and 'do wanted', showed gender-related DIF.

CONCLUSIONS: In the EQ-HWB, we found a few indications that people with limited cognitive capacities had difficulties in understanding the items. The conceptual framework of the EQ-HWB found in this sample can be described with the dimensions ‘psychological well-being’, ‘physical functioning’, and ‘physical sensation’