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When a screening test is done and a disease or precursor is picked up then an intervention has to be applied to change the course of the disease. If this intervention is not applied, or if it doesn’t work, than the individual has been identified as having disease earlier than they would have if symptoms had naturally developed. However, they would still die of advanced disease at the same age. Screening has not altered the course of their disease, but it has led to an earlier diagnosis. This is referred to as lead time bias. The graph illustrates this. The yellow line represents the length of time that an individual who had disease diagnosed when symptoms develop would have lived, the red part adds on the lead time introduced by early detection of disease through screening. The person lives longer with knowledge of the disease without any change in its natural history.
Screening for lung cancer with chest X-rays is an example of lead time bias. While tumours can be detected earlier, available treatments do not usually prolong life