What is screening?

What screening is

Screening applies a test to apparently healthy people to identify those who are more likely to have a disease or condition, so that further investigation or action can follow. A screening test is not diagnostic: a positive result indicates higher likelihood and is normally followed by a confirmatory diagnostic test. Screening is classed as secondary prevention, because it aims to detect disease early in its course rather than to stop it arising. This page treats screening as a population-level methodology and a research-standards topic; it does not advise on whether any individual should be screened.

Measuring test performance

A screening test is judged partly by how well it classifies people. Sensitivity is the proportion of those with the disease whom the test correctly identifies (true positives); specificity is the proportion without the disease whom it correctly clears (true negatives). There is usually a trade-off between them set by the test threshold.

The positive predictive value — the chance that a positive result is a true case — depends not only on sensitivity and specificity but on how common the disease is. When a condition is rare, even a good test yields many false positives, which is a central reason screening can cause harm as well as benefit.

Lead-time, length and over-diagnosis

Evaluating whether screening helps must allow for several biases. Lead-time bias arises because detecting disease earlier moves the diagnosis date forward, lengthening apparent survival even if the time of death is unchanged. Length(-time) bias arises because slower-progressing cases are more likely to be caught by periodic screening, making screen-detected disease look more survivable. Over-diagnosis is the detection of disease that would never have caused symptoms or harm in a person’s lifetime, leading to needless labelling and intervention. Because of these biases, improved survival among screen-detected cases does not by itself prove screening works; randomised evidence on mortality is the stronger standard.

Principles for screening programmes

Whether a population screening programme is worthwhile is assessed against established criteria, classically those set out by Wilson and Jungner for the WHO (1968) and since updated. These ask, among other things, that the condition be an important health problem with a recognisable early stage, that a suitable and acceptable test exist, that effective action follow a positive result, and that the benefits outweigh the harms and costs. The framing makes clear that screening is a programme-level decision balancing benefit against harm across a population — a standards and policy question, addressed here conceptually rather than as personal guidance.