Screening for Disease — Concepts, Criteria, Types

Screening is defined as 'the search for unrecognised disease or defect by means of rapidly applied tests, examinations, or other procedures in apparently healthy individuals.' The key word is 'apparently' — screening is done on people who do not have symptoms of the disease being screened for. Symptomatic individuals are diagnosed, not screened.

The aim of screening is to detect disease at an earlier stage than it would otherwise be detected, when treatment is more effective, less invasive, or less costly. Screening is one of the two pillars of secondary prevention (the other being early treatment). It is fundamentally different from primary prevention (which prevents disease occurrence) and tertiary prevention (which prevents disability from established disease).

• Mass screening: Screening the whole population regardless of risk. Example: cervical cancer screening for all women aged 30-65.
• Selective (targeted) screening: Screening high-risk groups only. Example: HBsAg screening in healthcare workers, HIV screening in TB patients, oral cancer screening in tobacco users.
• Multiphasic screening: Using multiple tests simultaneously to screen for several diseases. Example: Master Health Check-up packages.
• Case finding: Screening patients who present to a healthcare facility for any reason. Example: BP measurement of every adult visiting a clinic.
• Opportunistic screening: Screening performed when a patient happens to visit a healthcare provider.

The WHO Joint WHO/PHA Expert Committee on Screening (1968) laid down 10 principles to guide whether a disease warrants screening:

1. The condition should be an important health problem
2. There should be an accepted treatment for patients with recognised disease
3. Facilities for diagnosis and treatment should be available
4. There should be a recognisable latent or early symptomatic stage
5. There should be a suitable test or examination
6. The test should be acceptable to the population
7. The natural history of the condition, including development from latent to declared disease, should be adequately understood
8. There should be an agreed policy on whom to treat as patients
9. The cost of case-finding (including diagnosis and treatment of patients diagnosed) should be economically balanced in relation to possible expenditure on medical care as a whole
10. Case-finding should be a continuing process and not a 'once and for all' project

• Lead time bias: Screening detects disease earlier, so screened patients appear to survive longer from diagnosis — but their actual time of death is unchanged. Apparent survival benefit without real benefit.
• Length time bias: Screening preferentially detects slow-growing, less aggressive disease (because it remains in detectable pre-clinical phase longer). Screen-detected cancers therefore appear less aggressive than symptomatic cancers — overestimating survival benefit.
• Overdiagnosis bias: Screening detects indolent disease that would never have caused symptoms or death in the patient's lifetime. This inflates incidence without reducing mortality.
• Self-selection (volunteer) bias: People who participate in screening tend to be more health-conscious, educated, and affluent — so they have better outcomes regardless of screening.

Screening is a powerful public health tool but is not a substitute for diagnosis. Every screening programme must satisfy the Wilson-Jungner criteria and be evaluated for biases that can mislead clinicians, policymakers, and patients. For UPSC CMS aspirants, the Wilson-Jungner criteria and the three biases (lead time, length time, overdiagnosis) are extremely high-yield PSM topics.