Consistency matters: measurement invariance of the EORTC QLQ-C30 questionnaire in patients with hematologic malignancies.

To ensure that observed differences in the scores of the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire Core 30 (EORTC QLQ-C30) reflect actual differences in health-related quality of life (HRQoL) rather than measurement bias, measurement invariance needs to be established. We investigated the assumption of measurement invariance of the EORTC QLQ-C30 in patients with hematological malignancies across age, sex, comorbidity, disease type, and time. We used a large database of patients with hematological malignancies, which included HRQoL data collected with the EORTC QLQ-C30. We used the structural equation modeling approach to test for measurement (metric and scalar) invariance across groups (age, sex, comorbidity, disease) and time (baseline, 1 month and 2 month follow-up). Longitudinal invariance was examined in a subgroup of patients diagnosed with myelodysplastic syndromes. Confirmatory factor analyses demonstrated full measurement invariance for age and comorbidity and over time, while support for partial scalar invariance was obtained for sex and disease. Violations of invariance for sex were observed for items of the physical functioning scale and the emotional functioning scale, while for disease type, violations of invariance were observed for items of the physical functioning scale, emotional functioning scale, and the cognitive functioning scale. Our findings support measurement invariance of the EORTC QLQ-C30 in a large sample of patients with hematological malignancies. The results showed that the number of non-invariant items was negligible, suggesting that this questionnaire is a valid and robust measurement tool in patients with hematological malignancies, also for comparisons across groups and time.