Objective: This study aimed to evaluate the conceptual structure of the European Organization for Research and Treatment of Cancer Quality of Life Core Questionnaire 30 (EORTC QLQ-C30) by analyzing data collected from patients with major cancers in Taiwan. The conceptual structure underlying QLQ-C30, including higher-order factors, was explored by structural equation modeling (SEM). Methods: The Taiwan Chinese version of the EORTC QLQ-C30 was used as the measuring instrument. Higher-order models, including mental health/physical health, mental function/physical burden, symptom burden/function, single latent health-related quality of life, formative symptom burden/function, and formative health-related quality of life, were tested. Results: Study subjects included 283 patients with breast, lung, and nasopharyngeal cancers. The original QLQ-C30 multi-factorial structure demonstrated poor composite reliability of the cognitive function subscale. The formative symptom/burden model was favored by model fit indices, further supporting causal–indicator duality, but was compromised by unexpected associations between symptomatic subscales and latent factors. The formative health-related quality of life was proposed with a single second-order latent factor where symptomatic subscales remained formative. Two additional symptom measures from the formal cognitive function subscale with the formative health-related quality-of-life model were proposed as the alterative conceptual structure for the Taiwan Chinese QLQ-C30. Conclusions: Results of the current study represent the complete SEM approach for the EORTC QLQ-C30. The formative health-related quality-of-life model with elimination of cognitive function enhances the conceptual structure of the Taiwan Chinese version with parsimonious fit and interpretability.
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