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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
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