Definition and purpose
Screening is the application of a test to apparently healthy individuals to identify those with a higher probability of having a disease (or risk factor), so they can be offered confirmatory diagnosis and early intervention.
Purpose:
– reduce morbidity and/or mortality
– detect disease early (before symptoms)
– identify high-risk individuals
Screening vs diagnosis
- Screening test: done on apparently healthy people; not definitive; separates into “screen positive/negative”.
- Diagnostic test: done to confirm disease; higher certainty; used in symptomatic or screen-positive individuals.
Principles (Wilson–Jungner style)
A screening programme is justified when:
1. The condition is an important health problem.
2. There is a detectable early stage / latent phase.
3. A suitable, acceptable test exists.
4. Facilities for confirmatory diagnosis and treatment are available.
5. Early treatment is better than late treatment.
6. Benefits outweigh harms and costs.
7. There is an agreed policy on whom to treat.
8. Screening is continuous, not one-off (where relevant).
Harms and pitfalls
Common harms:
– False positives → anxiety, unnecessary procedures
– False negatives → false reassurance, delayed care
– Overdiagnosis and overtreatment
– Opportunity cost (money/time diverted from higher-impact interventions)
Biases:
– Lead-time bias: earlier detection appears to increase survival time without changing time of death.
– Length-time bias: slower-progressing cases more likely detected, making outcomes look better.
Example: Hypertension screening (practical)
- Target: adults ≥ 30 years (programme dependent)
- Test: BP measurement using validated device
- Screen positive: repeat measurement + confirm with standard protocol
- Follow-up: counselling, treatment initiation, referral pathway, adherence support
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