When it comes to survival comparison studies, researchers need to be aware of a crucial bias that can throw off their results: immortal time bias (ITB). But what exactly is ITB, and how can you avoid it in your study design?
ITB occurs when a study compares the survival rates of two groups, but the time of treatment or testing is not properly accounted for. This can lead to biased results, as the groups are not truly comparable.
Let’s take a closer look at two study designs that may be at risk of ITB.
**Study 1: Comparing Survival Rates with and without Mutation X**
In this study, all patients receive the same therapy, treatment A, after disease diagnosis. Some patients undergo genetic testing to determine whether they have mutation X or not. The key question is: would comparing the overall survival rates of patients with and without mutation X introduce ITB?
The answer lies in how the study handles patients who die or don’t get tested. If these patients are removed from the study, it could create an ITB problem. Why? Because the patients who are tested and found to have mutation X may have a different survival time than those who aren’t tested or don’t have the mutation.
**Study 2: Comparing Treatment A and B in Patients with Mutation X**
In this study, patients receive either treatment A or B, and all patients undergo genetic testing to determine whether they have mutation X or not. The question is: would comparing the overall survival rates of patients with mutation X who receive treatment A or B introduce ITB?
Again, the answer depends on how the study handles patients who don’t receive testing before treatment. If the study only includes patients who received testing before treatment, it may introduce ITB.
**The Bottom Line**
ITB is a real concern in survival comparison studies. To avoid it, researchers need to carefully consider their study design and ensure that they’re comparing apples to apples. By understanding the potential pitfalls of ITB, you can design a study that provides accurate and reliable results.
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*Further reading: [What is Immortal Time Bias?](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3151429/)*