“Back to the future”: Influence of beliefs regarding the future on TTO answers
BACKGROUND Quality-adjusted life-year (QALY) calculations in economic evaluations are typically based on general public or patient health state valuations elicited with the time tradeoff method (TTO). Such health state valuations elicited among the general public have been shown to be affected by respondents' subjective life expectancy (SLE). This suggests that TTO exercises based on time frames other than SLE may lead to biased estimates. It has not yet been investigated whether SLE also affects patient valuations. OBJECTIVE To empirically investigate whether patients' SLE affects TTO valuations of their current health state. METHODS Patients with different severities of diabetic retinopathy were asked in a telephone interview to value their own health status using TTO. The TTO time frame (t) presented was based on age- and sex-dependent actuarial life expectancy. Patients were then asked to state their SLE. Simple and multiple regression techniques were used to assess the effect of the patients' SLE on their TTO responses. RESULTS In total, 145 patients completed the telephone interview. Patients' TTO values were significantly influenced by their SLE. The TTO value decreased linearly with every additional year of difference between t and the patients' SLE; that is, patients were more willing to give up years the shorter their SLE compared with t. CONCLUSION Patients' SLE influenced their TTO valuations, suggesting that respondents' SLE may be the most appropriate time frame to use in TTO exercises in patients. The use of other time periods may bias the TTO valuations, as the respondents may experience the presented time frame as a gain or a loss. The effect seems to be larger in patient valuations than in general public valuations.