QALYt

QALYt is a proposed extension of the quality-adjusted life year (QALY) metric that explicitly incorporates time as a variable. In this framework, the health utility u(t) is allowed to vary over a specified time horizon, capturing changes in health status due to disease progression, treatment effects, adverse events, and recovery. The t in QALYt denotes time, making the measure sensitive to when quality of life changes occur within the evaluation period. As a theoretical concept, QALYt aims to support decision making for interventions with dynamic health trajectories.

Calculation of QALYt typically involves integrating the time-dependent utility over the chosen horizon. In continuous form, QALYt = ∫_0^T u(t) dt, and in discounted analyses, QALYt = ∫_0^T e^{-rt} u(t) dt, where r is the discount rate. In discrete models, the integral is approximated by a sum of utilities at successive time points multiplied by the corresponding time intervals. When u(t) is constant, QALYt reduces to the standard QALY.

Data and modeling for QALYt rely on longitudinal quality-of-life measurements and methods to map health states to time-varying utilities. Approaches include Markov or microsimulation models with time-dependent utilities drawn from instruments like EQ-5D or similar health-related quality-of-life measures. Modeling challenges include data availability, choice of time step, and assumptions about how utilities evolve between measurements.

QALYt has potential applications in chronic diseases, oncology, and conditions with fluctuating health states, but it remains a topic of methodological discussion rather than a standard metric. Its use requires clear specification of time horizon, discounting, and utility trajectories to ensure comparability with traditional QALY analyses.

See also: quality-adjusted life year, health economics, cost-utility analysis, health-state utilities, discounting.