Screening may be undertaken proactively, by systematically inviting everyone in a specified target population for screening over a defined time period. The UK breast screening programme, for example, invites all women aged 50–69 years for a screening test (mammography) once every 3 years.
Screening may also be undertaken opportunistically, when a person seeks help for another problem. Those attending emergency departments following an injury, for example, may be screened using a questionnaire to determine whether they are drinking alcohol at a hazardous level.
A population-based screening programme consists of more than a screening test (see Figure 33a for a generic flow diagram of any screening programme and Figure 33b for a simplified flow diagram for the UK breast screening programme). Follow-up investigations for those who test positive must be defined and available within a reasonable time. Effective treatment must be available for those who remain positive after further investigation. Staff must be trained to perform screening tests and further investigations, provide appropriate evidence-based advice on risks and benefits, and deliver treatment for those with the condition. The information conveyed to participants in a genetic screening programme requires particularly careful communication and provision of supporting advice (e.g. genetic counselling). Genetic test results may also have disease risk or reproductive planning implications for other family members, not just the person screened.
There must be explicit quality standards for delivery of the programme and an administrative system for inviting people to be screened, communicating results and referring on appropriately.