Lead-time bias
- Bias
Screening moves the date of diagnosis earlier; survival looks longer even if the person dies on the same date.
What it is
Lead-time bias arises when screening finds a disease before symptoms and survival is counted from diagnosis. The clock starts earlier, so screen-detected cases seem to live longer, even if treatment does not change the date of death. That is why a screening programme's efficacy is measured by disease-specific mortality in a randomised trial, not by case survival.
How it arises
- The survival clock starts on the date of diagnosis.
- Screening moves that date back, into the subclinical stage.
- If early treatment does not change the course, the date of death stays the same and the whole apparent gain is lead time.
Worked example
- Data (made up): a person's disease begins at age 50; symptoms would appear at 60 and death at 64.
- Without screening: diagnosis at 60; survival from diagnosis: 64 − 60 = 4 years.
- With screening: diagnosis at 57; if treatment changes nothing, death is still at 64: survival 64 − 57 = 7 years.
- Lead time: 60 − 57 = 3 years; the survival gained (7 − 4 = 3 years) is exactly the lead time.
- Five years after diagnosis, the unscreened person has died and the screened one is alive: five-year survival goes from 0 % to 100 %.
- In a trial of 20 000 people, 10 000 per arm, 30 die of the disease in each arm: 3 per 1 000 in both.
- Reading: survival improved through lead time alone; equal mortality shows that screening saved no lives.
Markers
- Lead time
- Five-year survival
- Disease-specific mortality
How to prevent or control it
- Design randomised trials that compare disease mortality between those invited and not invited to screening.
- If survival is used, count it from a date common to both groups (for example, randomisation) and not from diagnosis.
- Correct for the estimated lead time when comparing screen-detected and symptom-detected case series.
Common pitfalls
- Celebrating a rise in five-year survival after a screening programme starts without looking at mortality.
- Comparing screen-detected with symptom-detected cases as if they were equivalent groups.
Test yourself
Think of the answer, then tap the question to see it.
What is lead time?
The interval between detection of the disease by screening and the date it would have been diagnosed from symptoms.
Why is five-year survival no good for judging screening?
Because the clock starts earlier in screened people: survival rises even if the date of death does not change.
Which end point avoids lead-time bias when evaluating screening?
Disease-specific mortality in a randomised trial comparing those invited and not invited.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Cancer Screening Overview (PDQ®)–Health Professional Version (National Cancer Institute, 2023); Principles of Epidemiology | Lesson 1 - Section 9 (Centers for Disease Control and Prevention, 2012).
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Representative model, for educational purposes only.