Screening Programmes

Screening is the widespread use of a simple test for disease in an apparently healthy (asymptomatic) population. It is an example of secondary prevention — intervening after disease has begun but before symptoms appear, during the lead time: the interval between when the disease is detectable by screening and when it would present symptomatically.

A screening programme is an organised system using a screening test among asymptomatic people in the population to identify early cases for further diagnostic testing and treatment. The screening test is distinct from a diagnostic test: it is cheaper, simpler, and applied to large numbers; a positive screening test requires confirmation with a diagnostic test. Screening is a pathway, not a single test.

Screening aims to limit the consequences of disease through early diagnosis and treatment — ideally reducing mortality and/or morbidity. For screening to be justified, it must improve the length and/or quality of someone's life.

All screening programmes do harm; some also do good. Harms include: health inequities from unequal participation or treatment; physical harms from complications of tests and treatments (especially with false positives leading to unnecessary invasive procedures or false negatives causing delayed presentation); psychological harms from anxiety, distress, and prolonged awareness of diagnosis; and financial costs to individuals and health services.

Four considerations for deciding whether to screen:

(1) Appropriateness of the disease: serious enough to warrant screening; sufficient prevalence of pre-clinical disease; long enough lead time to allow benefit from early detection; an effective treatment available once detected.

(2) Appropriateness of the test: adequate sensitivity and specificity; safe and acceptable; simple and cheap.

(3) Effectiveness of the programme: evidence from RCTs of screening in other populations; capacity to manage positives (treatment resources, follow-up systems, quality control); cost-effectiveness.

(4) Benefits of screening: reduced mortality/morbidity; reassurance for true negatives; improved population health.

Three specific problems: lead time bias (early detection creates a false impression of prolonged survival without actually extending life — it moves the diagnosis earlier but not the death date); length bias (screening preferentially detects slowly progressing, less aggressive disease, giving a falsely favourable prognosis); overdiagnosis (detecting disease that would never have caused symptoms or death, leading to unnecessary treatment).

Prevention has three levels: primary (prevent disease occurring — reduce incidence); secondary (reduce impact by early detection and treatment — reduce duration/severity); tertiary (reduce complications and improve rehabilitation after disease). Prevention strategies include high-risk (individual) strategies and population (mass) strategies. The prevention paradox: a large number of people at small risk may give rise to more cases than a small number at high risk.