Terminology 101: Bias in screening
Bias in screening: A systematic error during the implementation of a screening program and/or the interpretation of its results that can lead to false conclusions about the benefits of the program
“Cancer screening saves lives.” It’s a message we’re all familiar with. But although screening helps detect disease early, and thus increases the chances of better disease management, the claim that screening by itself saves lives is difficult to support. That is because various types of bias can lead to an overestimation of survival benefits for individuals who participate in screening programs.
The first type is volunteer (or self-selection) bias. Individuals who participate in screening programs tend to have higher socio-economic status than those who do not, and they tend to adhere better to therapy. They may live longer than those who do not undergo screening for these same reasons rather than because of the benefits of screening.
The second type is lead-time bias. Lead time is the time from diagnosis by screening (before symptoms appear) to symptom manifestation. Screening might appear to prolong survival times, even if early treatment does not actually delay death, simply because individuals who undergo screening spend more time aware that they have the disease than people who only learn they have it after symptoms appear. To illustrate this point, let us consider two individuals who died of colon cancer in 2015. The first was diagnosed with the disease after a colonoscopy in 2010 once he presented with severe weight loss and unexplained rectal bleeding; the second was diagnosed after routine screening with fecal occult blood testing in 2006. The second person could be said to have lived longer with his cancer (to have survived longer) than the first person, but this is only because his disease was detected earlier.
The third type is length-time bias, which stems from the fact that the same disease may progress at different rates in different individuals. Length time is the amount of time it takes for a disease to progress to a particular end point (e.g., symptom manifestation or death). Given that slow-growing (or less aggressive) forms of a disease tend to have a longer asymptomatic phase than fast-growing (or aggressive) forms, there is a greater window of time when the former can be detected through screening. Screening may appear to improve survival times simply because it tends to pick up slow-growing forms of disease, which usually have more favourable prognoses than fast-growing ones.
In summary, although screening programs may help to improve survival by enabling clinicians to detect disease in its early stages and initiate timely treatment, it is important to be aware that other factors may contribute to the apparent survival benefits reported for screening initiatives. The best way to examine the benefits of screening is to conduct a randomized controlled trial, because the random assignment of participants to the screened and unscreened groups in a trial removes the distorting effects of volunteer, lead-time and length-time bias.
Nurse.ONE.ca resources on this topic
- Marczyk, G. R., DeMatteo, D., & Festinger, D. (2005). Essentials of Research Design and Methodology.
- Newell, R., & Burnard, P. (2011). Research for Evidence-Based Practice in Healthcare.
- Supino, P. G., & Borer, J. S. (Eds.). (2012). Principles of Research Methodology: A Guide for Clinical Investigators.
- Waltz, C. F., Strickland, O. L., & Lenz, E. R. (2010). Measurement in Nursing and Health Research.