What is the immortal time bias?
Such studies may be subject to immortal time bias, meaning that, during the period of observation, there is some interval during which the outcome event cannot occur. The research participants are “immortal” in that they must survive long enough to receive the intervention being studied.
Is immortal time bias a form of selection bias?
Immortal time refers to a period of follow-up during which, by design, death or the study outcome cannot occur. This is a frequent problem in observational pharmacoepidemiology studies and is often due to selection bias, although we will see in the next section that misclassification can also explain this bias.
What is guarantee time bias?
Guarantee-time bias means that the participants in the treatment arm will only be monitored/followed up if they survive (or, for example, do not have symptoms that the treatment aims to prevent) for the whole of the time lag.
What is an example of information bias?
Missing data can be a major cause of information bias, where certain groups of people are more likely to have missing data. An example where differential recording may occur is in smoking data within medical records. The bias was more likely when the exposure is dichotomized.
What is immortal time?
Immortal time is a span of cohort follow-up during which, because of exposure definition, the outcome under study could not occur. Bias from immortal time was first identified in the 1970s in epidemiology in the context of cohort studies of the survival benefit of heart transplantation.
How can you control your immortal time bias?
Immortal time bias can be avoided by acknowledging a change in exposure status using a time-dependent covariate. For example, a MDC clinic patient would be considered unexposed from the date of study entry until he or she visits the MDC clinic and exposed from that point forward.
How do you avoid immortal time bias?
The only way to prevent immortal time bias is by designing studies so that participants are assigned to exposure groups based on their data at time-zero, rather than their data after time-zero. In other words, assignment and time-zero must be aligned.
What is lead time bias in epidemiology?
A distortion overestimating the apparent time surviving with a disease caused by bringing forward the time of its diagnosis.
What is collider bias?
A distortion that modifies an association between an exposure and outcome, caused by attempts to control for a common effect of the exposure and outcome.
Is there a problem with immortal time bias?
Immortal time in observational studies can bias the results in favour of the treatment group, but it is not difficult to identify and avoid Well designed observational studies have made important contributions to our understanding of the risks and benefits of drug treatment.
What is the hazard ratio of immortal time?
A proper time-dependent analysis that correctly classified immortal person-time to the untreated group reversed the effect and revealed that the hazard ratio was 1.97 (1.53 to 2.52). This reanalysis also showed that the magnitude of immortal-time bias increases as the duration of immortal time increases. [4]
How does immortal time bias occur in exposure-based cohorts?
Exposure-based cohorts. Thus, immortal time bias occurs because valid bronchodilator person-time of follow-up with no deaths is not accounted for in the reference rate of death, resulting in an artificial increase in the rate of death in the reference group, leading to a spurious appearance of effectiveness ( 10 ).
How does immortal time bias affect insulin risk?
In contrast, the immortal time corrected crude rate ratio was 2.68. Similarly, statin users seemed to be at lower risk of progressing to insulin in the time fixed Cox analysis (adjusted hazard ratio 0.74, 95% confidence interval 0.58 to 0.95) but not in the time dependent analysis (1.97, 1.53 to 2.52).