Overdiagnosis
- Bias
Overdiagnosis is diagnosing a real disease that would never have caused symptoms or death in that person's lifetime.
What it is
Overdiagnosis happens when screening or a test finds a true disease (not a false positive) that would not have caused symptoms or shortened life, because it grows very slowly or because the person dies first of something else. The patient gains nothing and suffers the harms of treatment, the label and anxiety. It is the extreme form of length-time bias and one of the reasons for quaternary prevention.
How it arises
- Lesions that grow very slowly or do not progress.
- Older people or people with other diseases who die first of another cause.
- Ever more sensitive tests and lower diagnostic thresholds that turn small findings into disease.
Worked example
- Data (made up): a trial assigns 10 000 men to yearly screening for a slow-growing cancer and 10 000 to control.
- At 10 years: 300 diagnoses in the screened arm and 200 in the control arm.
- Follow-up to 15 years: the control arm reaches 220 cases and the screened arm stays at 300; control never catches up.
- Persistent excess: 300 − 220 = 80 cancers that appeared only because of screening.
- Share overdiagnosed among screened cases: 80 / 300 = 0.267, that is, ≈ 27 %.
- Deaths from that cancer at 15 years: 30 in the screened arm and 32 in the control arm; a difference of 2 per 10 000.
- Reading: for every 2 deaths avoided there were 80 extra diagnoses; those men were treated with no benefit.
Markers
- Overdiagnosis
- Overtreatment
- Quaternary prevention
How to prevent or control it
- Choose the screening population and interval carefully, and do not screen people with a short life expectancy.
- Offer active surveillance instead of immediate treatment for low-risk lesions when the evidence supports it.
- Explain in shared decision-making that screening may find disease that would not have caused harm.
Common pitfalls
- Mixing up overdiagnosis and false positives: in overdiagnosis the disease is real.
- Counting the overdiagnosed as lives saved because they survive after treatment.
Test yourself
Think of the answer, then tap the question to see it.
How does overdiagnosis differ from a false positive?
In a false positive the person does not have the disease; in overdiagnosis they do, but it would not have caused symptoms or death.
How is overdiagnosis estimated in a screening trial?
As the excess of cases in the screened group over the control group that persists after long follow-up.
Which pattern of incidence and mortality suggests overdiagnosis in a population?
Incidence rising sharply when screening arrives while mortality from that disease stays the same.
Which level of prevention is linked to controlling overdiagnosis?
Quaternary prevention: protecting people from interventions that bring them no benefit.
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); Screening programmes: a short guide. Increase effectiveness, maximize benefits and minimize harm (World Health Organization, Regional Office for Europe, 2020).
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Representative model, for educational purposes only.