Objective: To adjust patients' time trade-off (TTO) scores
using information on their utility functions for survival time to derive a
measure of health state utility equivalent to the standard gamble (SG).
Methods: A sample of 199 cardiovascular patients were asked three
TTO and SG questions (to assess their own health state), and three certainty
equivalent questions (to assess their utility function for survival time) in
an interview.
Results: Patients' utility functions for time were
increasingly concave, but being unable to model this successfully, a constant
function with an averaged level of concavity was used. The raw TTO scores were
significantly higher than SG scores, while the adjusted TTO scores were
equivalent to the SG.
Conclusions: Raw time trade-off scores will give biased estimates
of health state utility when patients' utility functions for time are not
linear, but these can be adjusted to yield true utilities. The constant
proportional risk-posture assumption of the conventional QALY model, on which
previous attempts to adjust time trade-offs have been based, was not supported
by the data.