Principles of screening
- Concept
Screening looks for disease in people without symptoms; it is worth it only if the benefit is real and the harms are smaller.
What it is
Screening applies a test to people without symptoms to identify those who probably have a disease at an early stage. A positive result is not a diagnosis: it needs confirmation. A screening programme is justified only if the disease is important and has a detectable preclinical stage, the test is acceptable and valid, early treatment improves the outcome, and the whole programme (invitation, confirmation, treatment, follow-up) works and is evaluated.
Worked example
- Data (made up): 10 000 adults are screened; the prevalence of preclinical disease is 1 % (100 people).
- The test has 90 % sensitivity and 95 % specificity.
- True positives: 0.90 × 100 = 90; false negatives: 100 − 90 = 10.
- Healthy: 10 000 − 100 = 9 900; true negatives: 0.95 × 9 900 = 9 405; false positives: 9 900 − 9 405 = 495.
- All positives: 90 + 495 = 585; positive predictive value: 90 / 585 = 0.154, that is, ≈ 15 %.
- Negative predictive value: 9 405 / (9 405 + 10) = 9 405 / 9 415 ≈ 99.9 %.
- Reading: of every 100 positives, about 85 are healthy; the programme must offer a confirmatory test and count the harms of false positives.
What each component must meet for screening to be worth it
| Component | Criterion | If it fails |
|---|---|---|
| Disease | Important, with a detectable preclinical stage | Nothing to find early |
| Test | Valid, safe, acceptable, affordable | False positives and negatives, low uptake |
| Treatment | Better when early | Diagnosis is moved earlier with no benefit (lead time) |
| Programme | Accessible confirmation and treatment, ongoing evaluation | Positives with no follow-up, harm without benefit |
Markers
- Screening
- Case finding
- PPV
- Preclinical stage
In practice
- In Colombia, the health promotion and maintenance pathways set which screening tests are offered by age and sex.
- Explaining that a positive screening test is not a diagnosis reduces anxiety and improves uptake of confirmation.
Common pitfalls
- Judging a programme by the survival of detected cases: lead time, length-time bias and overdiagnosis inflate it.
- Screening without the capacity to confirm and treat: it creates positives that nobody follows up.
- Forgetting the harms: false positives, invasive procedures, overdiagnosis and anxiety.
Test yourself
Think of the answer, then tap the question to see it.
How does population screening differ from case finding?
Screening invites a whole target population without symptoms; in case finding the clinician tests someone who came for another reason.
Why does screening a low-prevalence population produce many false positives?
Because there are many more healthy than ill people: a small percentage of false positives among many healthy people outnumbers the true positives.
What must hold for treatment if screening is to make sense?
It must work better when given at the early stage than when waiting for symptoms.
Is a positive screening result a diagnosis?
No: it identifies a person with a higher probability of having the disease, who needs a confirmatory test.
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: Screening programmes: a short guide. Increase effectiveness, maximize benefits and minimize harm (World Health Organization, Regional Office for Europe, 2020); Cancer Screening Overview (PDQ®)–Health Professional Version (National Cancer Institute, 2023).
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Representative model, for educational purposes only.