Abstract
Purpose: To propose the Entry-time Comparability Retention (ETCR)
method- a novel methodology that uses propensity scores (PS) as a design
tool to minimize immortal time bias- a bias that occurs when the
determination of a patient’s treatment status involves a period prior to
treatment initiation that is unaccounted for in the assessment of
treatment effect. Methods: The treatment status of each member of a
cohort is classified at cohort entry, based on whether or not the
patient initiated treatment at any time during the study period- namely,
as either ever-treated or never-treated. Two sets of propensity scores
are derived on each patient: 1) at cohort entry to construct blocks of
comparable patients by propensity scores matching each ever-treated
patient to the never-treated patients who survived up to the
ever-treated patient’s time of treatment initiation, which are then
retained and assigned the same start of follow-up to address the
immortal time bias, and 2) at treatment initiation, to derive the second
set of propensity scores to address confounding. Using simulated data,
the performance of a simplified form of the ETCR method, that is based
on simple blocks of 1:n greedy matching, is compared with two of the
leading approaches- the Landmark and time-dependent Cox methods.
Results: The ETCR method consistently yielded the lowest mean percentage
of residual bias across the different scenarios in our simulation.
Conclusions: The ETCR results suggest the propensity scores as an
effective design tool for creating and retaining comparable treatment
groups to minimize immortal time bias.