Slide 1
Earlier discovery changes the starting clock
Screening looks for disease before symptoms lead to diagnosis. Finding it earlier can increase measured survival after diagnosis even without postponing death.
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The measurement begins at diagnosis, so moving that date changes the interval being counted. This is called lead-time bias: a longer recorded interval need not mean a longer life.
Slide 2
A simple timeline exposes the effect
Imagine diagnosis at age 65 and death at 70: survival is five years. Diagnosis at 62 with the same death at 70 records eight years.
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This is an illustrative calculation, not clinical data: 70 minus 65 is 5, while 70 minus 62 is 8. The three extra recorded years come entirely from starting the clock earlier.
Slide 3
Some detected disease would never cause harm
Screening can detect a disease that would not have produced symptoms during a person’s lifetime. This is overdiagnosis, which differs from a false-positive test.
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In overdiagnosis the detected abnormality is real, but treating it may offer no benefit to that person. A false positive instead indicates suspected disease that later evaluation does not confirm.
Slide 4
Deaths and harms must also be compared
Evaluating screening requires outcomes beyond survival measured from diagnosis. Trials can compare disease-related mortality and the harms caused by screening and follow-up.
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Additional tests and treatment can create burdens even when an initial screen finds something early. Benefit and harm need the same careful evaluation rather than assuming that every earlier finding improves health.
Slide 5
The bias does not mean screening never helps
Some screening programs improve important outcomes; others may not. The evidence must show benefit for the particular test, population and schedule.
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A headline about better survival among screen-detected cases cannot by itself establish that benefit. The appropriate conclusion depends on comparative evidence that accounts for the biases introduced by detection.