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Background: A response shift is a psychological change in one's perception of the quality of life following a change in health status. This phenomenon initially was recognized in patients with terminal diseases who, despite a worsening of the physical condition, did not necessarily report deterioration in quality of life. The purpose of the present study was to examine the role of response shift in patients undergoing total knee replacement surgery.Methods: Consecutive candidates undergoing total knee replacement for the treatment of degenerative arthritis completed a Western Ontario and McMaster Universities Osteoarthritis questionnaire preoperatively (Pre-Test). At six months postoperatively, the patients completed two questionnaires: one on how they felt currently (Post-Test), and one on how they perceived themselves to have been prior to surgery (Then-Test). The study cohort comprised 125 subjects, including ninety-one women and thirty-four men, with a mean age (and standard deviation) of 68 ± 9.5 years. The impact of response shift was examined statistically.Results: With use of the Then-Test methodology, a significant (p < 0.05) response shift was observed in the domains of pain, physical function, and total Western Ontario and McMaster Universities Osteoarthritis score, indicating that patients perceived themselves as having been more disabled than what they had reported before surgery. In measuring outcome, this translates into the treatment effect being greater when adjusting for the presence of a response shift. With the numbers available, age, gender, comorbidity, and amount of recovery did not have a significant impact on response shift when adjusted for the preoperative level of disability.Conclusions: Patients who have undergone total knee replacement demonstrate a response shift in the measurement of their outcome at six months postoperatively. Although the response shift effect in the present study did not affect the interpretation of clinical results, we have highlighted the different patterns of individuals' psychological adaptation to a change in health status. This is an essential component of assessing the success or failure of surgical interventions as quantified with self-administered quality-of-life measures.Level of Evidence: Prognostic Level I. See Instructions to Authors for a complete description of levels of evidence.