Minimally important differences for interpreting EORTC QLQ-C30 scores in patients with advanced breast cancer

Background

We aimed to estimate the minimally important difference (MID) for interpreting group-level change over time, both within a group and between groups, for European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire core 30 (EORTC QLQ-C30) scores in patients with advanced breast cancer.

Patients and Methods

Data were derived from two published EORTC trials. Clinical anchors, e.g. performance status, were selected using correlation strength and clinical plausibility of their association with a particular QLQ-C30 scale. Three change status groups were formed: deteriorated by one anchor category, improved by one anchor category and no change. Patients with greater anchor changes were excluded. The mean change method was used to estimate MIDs for within-group change and linear regression was used to estimate MIDs for between-group differences in change over time. For a given QLQ-C30 scale, MID estimates from multiple anchors were triangulated to a single value via a correlation-based weighted average.

Results

MIDs varied by QLQ-C30 scale, direction (improvement versus deterioration) and anchor. MIDs for within-group change ranged from 5 to 14 points (improvement) and –14 to –4 points (deterioration), and MIDs for between-group change over time ranged from 4 to 11 points and from –18 to –4 points. Correlation-weighted MIDs for most QLQ-C30 scales ranged from 4 to 10 points in absolute values.

Conclusions

Our findings aid interpretation of changes in EORTC QLQ-C30 scores over time, both within and between groups, and for performing more accurate sample size calculations for clinical trials in advanced breast cancer.