Immortal time bias occurs when exposure time is misclassified in cohort studies; which description best fits it?

• A. Misclassification of exposure time in cohort studies where individuals must survive a period to be classified as exposed, artificially inflating survival time.
• B. A bias from recalling past exposures.
• C. A bias due to selection of only healthy participants.
• D. A bias introduced by loss to follow-up.

Answer: (C)

Immortal time bias shows up when the way exposure is assigned in a cohort study requires people to survive a period before they can be counted as exposed. That pre-exposure time is in effect counted as exposed person-time, even though the person wasn’t yet exposed. Because outcomes could not occur during that “immortal” interval, the exposed group ends up with artificially longer survival or lower apparent risk, simply due to how time is classified, not because the exposure truly protects against the outcome.

This is different from recall bias, which comes from inaccurate memory of past exposures; it’s different from selection bias from choosing only healthy participants; and it’s different from bias due to loss to follow-up. Immortal time bias specifically involves misclassification of exposure time and the resulting distorted time-at-risk.