Screening is usually considered a form of secondary disease prevention, as it aims to reduce morbidity and mortality from a disease, rather than preventing the disease altogether. The benefits of screening arise from early diagnostic testing and treatment compared with usual symptomatic (‘late’) diagnosis and treatment (Figure 32a). Some screening tests and programmes, however, pick up precursors of a disease, rather than the disease itself; treating the disease precursors may prevent the disease occurring in the first place. An example is screening for cervical cancer, where the screening test picks up precancerous changes that can then be treated, thus preventing progression to cancer. Other screening tests and programmes pick up conditions that cannot be prevented, particularly genetically determined ones such as Huntington’s disease, when the aim of screening is to provide information for those affected.
The UK National Screening Committee currently defines screening as follows:
A process of identifying apparently healthy people who may be at increased risk of a disease or condition. They can then be offered information, further tests and appropriate treatment to reduce their risk and/or any complications arising from the disease or condition.
Screening is generally thought to be a ‘good thing’ as it is appealing to think that, if a disease is picked up at an early stage, treatment is more likely to prevent serious illness developing and may lead to cure (Figure 32a). However, it is important to realise that this intuitive understanding of screening is based on several assumptions:
- that the natural history of the disease is well known, that it has a latent phase when it can be identified before the person is aware of any symptoms for which they would normally seek health advice, and that progression from that latent phase to serious disease is highly probable.
- that there is an appropriate test available to accurately identify those with latent disease and to exclude those without the disease.
- that effective treatment for early disease is available and that treatment at an early stage leads to better outcomes for the person compared with treatment following presentation with clinical symptoms.
- that no one will be harmed by the test, the treatment or the screening process.
Certain criteria must, therefore, be met before implementing any screening programme to ensure that it is likely to produce more benefit than harm (see Chapter 38).
Screening tests should ideally be very sensitive, so that very few people with the disease are missed (false negatives) and also very specific, so that very few people without the disease are identified for further investigation and treatment (false positives) (see Chapter 13). In reality, no screening test is perfect and there is a trade-off between sensitivity and specificity to be made. It is also important to remember that, when screening is carried out on an unselected general population, disease prevalence is usually low.
In the pilot programmes for bowel cancer screening in England, only 10% of those undergoing further investigation following a positive faecal occult blood (FOB) test were found to have bowel cancer (i.e. the positive predictive value of the FOB test for bowel cancer in the screened population is 10%).