Magnitude and correlates of response shift in fatigue ratings in women undergoing adjuvant therapy for breast cancer.

Fatigue is a common consequence of cancer chemotherapy and radiotherapy. Investigation of the impact of cancer treatment on fatigue is compromised by a potential "response shift" in fatigue ratings, involving a recalibration of a respondent's internal standard used to judge their current fatigue experience. Using a prospective, longitudinal research design, repeated assessments of most and average fatigue were obtained from women (n=102) with early stage breast cancer receiving adjuvant chemotherapy and/or radiotherapy. Then test ratings of fatigue were also obtained and used to index the presence of a response shift in fatigue ratings. Results suggested a clinically significant response shift in ratings of most and average fatigue of approximately one-half standard deviation. Multiple regression analyses suggested larger response shifts were associated with higher pretreatment fatigue ratings, receipt of chemotherapy, and lower fatigue catastrophizing. Most importantly, consideration of the observed response shift in fatigue ratings significantly altered the conclusions drawn regarding the magnitude and persistence of fatigue during and after adjuvant therapy for breast cancer.