Factor analysis of the psychosocial items of the EORTC QLQ-C30 in metastatic breast cancer patients participating in a psychosocial intervention study.

The underlying factor structure of a subset of 12 items, which comprise the psychosocial subscales of the EORTC QLQ-C30 was explored in a group of women, all with metastatic breast cancer who were participating in a psychosocial intervention study. Two main factors were identified in this exploratory factor analysis, representing "emotional distress" and "functional ability" dimensions. A preliminary assessment of the external validity of the two factor structure was undertaken. The results support the validity of a summative "emotional distress" and "functional ability" score in this sample of patients. The "functional ability" score discriminated well for subgroups defined by clinical status indicators (e.g., performance status, pain, chemotherapy treatment, fatigue). The "emotional distress" subscale discriminated with respect to suffering, fatigue and sleep disturbance. Both subscales converged with related concepts measured by independent instruments, providing support for convergent validity. Summative index scores may be advantageous for application in particular research situations; applying quality adjustments in health policy analyses; for screening purposes; to monitor populations and make comparisons across broad groups and as stratification variables in clinical trials. Further research to confirm the 2 factor structure is required in other samples before the interpretation can be accepted with confidence.