Impact of the occurrence of a response shift on the determination of the minimal important difference in a health-related quality of life score over time

BACKGROUND An important challenge of the longitudinal analysis of health-related quality of life (HRQOL) is the potential occurrence of a Response Shift (RS) effect. While the impact of RS effect on the longitudinal analysis of HRQOL has already been studied, few studies have been conducted on its impact on the determination of the Minimal Important Difference (MID). This study aims to investigate the impact of the RS effect on the determination of the MID over time for each scale of both EORTC QLQ-C30 and QLQ-BR23 questionnaires in breast cancer patients. METHODS Patients with breast cancer completed the EORTC QLQ-C30 and the EORTC QLQ-BR23 questionnaires at baseline (time of diagnosis; T0), three months (T1) and six months after surgery (T2). Four hospitals and care centers participated in this study: cancer centers of Dijon and Nancy, the university hospitals of Reims and Strasbourg At T1 and T2, patients were asked to evaluate their HRQOL change during the last 3 months using the Jaeschke transition question. They were also asked to assess retrospectively their HRQOL level of three months ago. The occurrence of the RS effect was explored using the then-test method and its impact on the determination of the MID by using the Anchor-based method. RESULTS Between February 2006 and February 2008, 381 patients were included of mean age 58 years old (SD = 11). For patients who reported a deterioration of their HRQOL level at each follow-up, an increase of RS effect has been detected between T1 and T2 in 13/15 dimensions of QLQ-C30 questionnaire, and 4/7 dimensions of QLQ-BR23 questionnaire. In contrast, a decrease of the RS effect was observed in 8/15 dimensions of QLQ-C30 questionnaire and in 5/7 dimensions of QLQ-BR23 questionnaire in case of improvement. At T2, the MID became ≥ 5 points when taking into account the RS effect in 10/15 dimensions of QLQ-C30 questionnaire and in 5/7 dimensions of QLQ-BR23 questionnaire. CONCLUSIONS This study highlights that the RS effect increases over time in case of deterioration and decreases in case of improvement. Moreover, taking the RS into account produces a reliable and significant MID.