Left truncation results in substantial bias of the relation between time-dependent exposures and adverse events.

Purpose: To assess the impact of random left truncation of data on the estimation of time-dependent exposure effects.
Methods: A simulation study was conducted in which the relation between exposure and outcome was based on an immediate exposure effect, a first-time exposure effect, or a cumulative exposure effect. The individual probability of truncation, the moment of truncation, the exposure rate, and the incidence rate of the outcome were varied in different simulations. All observations before the moment of left truncation were omitted from the analysis.
Results: Random left truncation did not bias estimates of immediate exposure effects, but resulted in an overestimation of a cumulative exposure effect and underestimation of a first-time exposure effect. The magnitude of bias in estimation of cumulative exposure effects depends on a combination of exposure rate, probability of truncation, and proportion of follow-up time left truncated.
Conclusions: In case of a cumulative or first-time exposure, left truncation can result in substantial bias in pharmacoepidemiologic studies. The potential for this bias likely differs between databases, which may lead to heterogeneity in estimated exposure effects between studies.
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