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. 2021 Oct;36(10):3372-3377.
doi: 10.1016/j.arth.2021.06.012. Epub 2021 Jun 21.

Immortal Time Bias in the Analysis of Time-to-Event Data in Orthopedics

Dirk R Larson  1 , Cynthia S Crowson  2 , Katrina L Devick  3 , David G Lewallen  4 , Daniel J Berry  4 , Hilal Maradit Kremers  5
Affiliations

Immortal Time Bias in the Analysis of Time-to-Event Data in Orthopedics

Dirk R Larson et al. J Arthroplasty. 2021 Oct.

Abstract

Many outcomes in arthroplasty research are analyzed as time-to-event outcomes using survival analysis methods. When comparison groups are defined after a time-delayed exposure or intervention, a period of immortal time arises and can lead to biased results. In orthopedics research, immortal time bias often arises when a minimum amount of follow-up is required for study inclusion or when comparing outcomes in staged bilateral vs unilateral arthroplasty patients. We present an explanation of immortal time and the associated bias, describe how to correctly account for it using proper data preparation and statistical techniques, and provide an illustrative example using real-world arthroplasty data. We offer practical guidelines for identifying and properly handling immortal time to avoid bias. Please visit the followinghttps://youtu.be/58p8w5o-ci4for a video that explains the highlights of the paper in practical terms.

Keywords: censoring; immortal time bias; survival analysis; time-to-event; total joint arthroplasty.

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Figures

Figure 1,
Figure 1,
Panel A: Inappropriate assignment of immortal time; Panel B: Inappropriate exclusion of immortal time
Figure 2:
Figure 2:
Proper assignment of follow-up time for exposed and unexposed subjects when immortal time is present
Figure 3,
Figure 3,
Panel A: Survival after initial THA for patients with bilateral THA (solid line) and unilateral THA (dashed line), demonstrating immortal time bias; Panel B: Survival after THA after correctly assigning follow-up time to the bilateral and unilateral THA groups.
See this image and copyright information in PMC

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