[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Screening](https://durgical.com/en/epidemiology/areas/screening/) › Principles of screening

# 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.

Updated Nov 5, 2026

## Key points

- The disease: an important health problem, with a known natural history and a detectable preclinical stage.
- The test: safe, acceptable, sensitive and specific, with an agreed cutoff.
- The treatment: more effective when given early than when symptoms appear.
- The programme: access to confirmation and treatment, a defined target population, reasonable costs and quality evaluation.
- Population screening (the whole target population is invited) versus case finding (the clinician tests someone who came for another reason).
- Even with a good test, when prevalence is low most positives are false positives.

## 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.

## Memory aid

Worth it if the disease matters, the test works, early treatment helps and the system responds: four yeses or no screening.

## 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.

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## Connections

## Related 6

- [Sensitivity](https://durgical.com/en/epidemiology/sensitivity/)
- [Specificity](https://durgical.com/en/epidemiology/specificity/)
- [How prevalence changes predictive values](https://durgical.com/en/epidemiology/how-prevalence-changes-predictive-values/)
- [Lead-time bias](https://durgical.com/en/epidemiology/lead-time-bias/)
- [Length-time bias](https://durgical.com/en/epidemiology/length-time-bias/)
- [Overdiagnosis](https://durgical.com/en/epidemiology/overdiagnosis/)

## Related conditions 3

- [Cervical cancer](https://durgical.com/en/conditions/cervical-cancer/)
- [Breast cancer](https://durgical.com/en/conditions/breast-cancer/)
- [Colorectal cancer](https://durgical.com/en/conditions/colorectal-cancer/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

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](https://www.who.int/europe/publications/i/item/9789289054782) (World Health Organization, Regional Office for Europe, 2020); [Cancer Screening Overview (PDQ®)–Health Professional Version](https://www.cancer.gov/about-cancer/screening/hp-screening-overview-pdq) (National Cancer Institute, 2023).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/principles-of-screening/