Underlying dimensions of the EORTC QLQ-C30 in a Cuban population of patients with advanced non-small cell lung cancer.

BACKGROUND

Quality of Life Core Questionnaire of the European Organization for the Research and Treatment of Cancer (EORTC QLQ-C30) is one of the most used quality of life questionnaires in cancer studies. It provides scores for five functional scales, nine symptom scales, and two single items which assess overall health status and quality of life. However, high correlations among QLQ-C30 items suggest a reduced dimensionality for the scale.

OBJECTIVE

To assess the dimensionality of the EORTC QLQ-C30 using item response theory (IRT) in a training sample and confirmatory factor analysis (CFA) in a test sample.

METHODS

We analyzed responses to QLQ-C30 from 1,107 patients with advanced lung cancer who were included in five clinical trials of immunotherapy. We used non-parametric and parametric IRT models (Mokken, and Samejima's graded response) in a random training set (n = 332) for initial assessment of dimensions and item characteristics of the QLQ-C30. Finally, we used CFA in the test set (n = 775) to confirm the measurement domains.

RESULTS

Mokken model showed that QLQ-C30 fits a unidimensional scale, whereas Samejima model showed that most QLQ-C30 items present adequate difficulty and discrimination. All items showed adequate scalability indexes with an overall scalability of 0.47 (medium scale). The QLQ-C30-reduced dimensionality was confirmed by CFA (comparative fit index = 0.98, root mean square error of approximation = 0.055) with all items presenting factorial loadings > 0.40.

CONCLUSIONS

The EORTC QLQ-C30 fits a unidimensional latent construct identified with perceived quality of life in advanced lung cancer patients.

TRIAL REGISTRATION

RPCEC00000161, RPCEC00000181 and RPCEC00000205.