Response shift and adaptation in chronically ill patients.

The purpose was to explore the impact of response shift on quality-of-life measurement and consequently on cost-effectiveness analysis. This was done by using data from an earlier study. Adaptation to illness provides a probable explanation for the occurrence of response shift. In the case of proxy respondents, it is likely that the adaptation effect is absent. Therefore, two hypotheses were tested: 1) retrospective quality-of-life measurement does not differ from ratings given by proxies; and 2) proxy ratings differ from prospectively assessed pretreatment quality-of-life scores. Quality of life was assessed using a visual analog scale (VAS), time tradeoff (TTO), and the standard gamble (SG). Analyses revealed that neither hypothesis could be rejected, indicating that prospectively assessed pretreatment quality of life is enhanced by adaptation to the imperfect health state during the pretreatment period. Consequently, the cost-effectiveness ratio is different when using proxy measures or retrospective assessments of pretreatment quality of life, compared with using assessments of the quality of life in currently ill patients.