Screening: introduction and policy context

Public information on screening programmes is provided by NHS Inform. These ScotPHO pages are for a professional audience and public health intelligence in relation to screening programmes.

The underlying concept of screening is that in some circumstances the early detection of risk factors or undiagnosed disease can be associated with health benefits. Screening is designed to identify conditions early, before symptoms are present or a diagnosis is made.

Screening has been defined as:

"a public health service in which members of a defined population, who do not necessarily perceive they are at risk of, or are already affected by a disease or its complications, are asked a question or offered a test, to identify those individuals who are more likely to be helped than harmed by further tests or treatment to reduce the risk of a disease or its complications" (UK National Screening Committee).

Or more simply:

"the use of simple tests across an apparently healthy population in order to identify individuals who have risk factors or early stages of disease, but do not yet have symptoms" (World Health Organization).

Screening differs to diagnostic testing (see HealthKnowledge) as it aims to detect early disease or risk factors for disease in apparently healthy individuals, while diagnostic testing seeks to establish the presence (or absence) of disease as a basis for treatment decisions in individuals who are symptomatic or who have screened positive.

When is screening appropriate?

Decisions on whether a screening programme should be introduced are based on the ten criteria described by Wilson and Jungner in 1968 (6Mb). These can be grouped as follows:

• The disease or condition:

1. Should be an important health problem.
2. The natural history of the condition, including development from latent to declared disease, should be adequately understood.
3. There should be a recognisable latent or early symptomatic stage.

• The screening test:

1. There should be a suitable test or examination.
2. The test should be acceptable to the population.
3. Case-finding should be a continuing process and not a 'once and for all' project.

• The diagnosis/treatment:

1. There should be an agreed policy on whom to treat as patients.
2. There should be an accepted treatment for patients with recognized disease.
3. Facilities for diagnosis and treatment should be available.

• The programme overall:

1. 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.

The criteria have been expanded for appraising the variability, effectiveness and appropriateness of a screening programme by the UK National Screening Committee. The Scottish Government advocates that the criteria set by the UK National Screening Committee should ideally be met before screening for a condition is initiated.

Other similar criteria have been proposed (see for example Harris et al (2011)) and there are debates about widening the criteria further with, for example, the sequencing of the human genome and identification of many more genetic conditions through their genetic code, see Andermann et al (2008).

Other developments since the publication of the Wilson-Jungner criteria include:

• Increased screening for risk factors (eg raised glucose or cardiovascular risk scores) rather than for disease.
• Increased use of screening under other labels such as “case finding”, “disease awareness” campaigns or health checks.
• Controversies about breast and prostate screening have increased the awareness of the need for high quality trials to support new screening programmes.
• Increased awareness of the potential for harm as well as concerns that those participating in screening have not been fully informed of possible harms.

Why is screening different to other health services?

Screening differs to other aspects of the health service as it targets and offers apparently healthy people help to make better informed choices about their health. There are, however, risks involved in screening. See Public Health England’s Guidance, NHS population screening explained.

Screening can reduce the risk of developing a condition or related complications but it cannot offer a guarantee of total protection from a particular illness. In any screening programme, there can be false positive results (wrongly reported as having the condition) and false negative results (wrongly reported as not having the condition). There are therefore opportunity costs and harms surrounding screening. For this reason, the ethical, economic, legal and social consequences should always be carefully considered prior to the introduction of a new screening programme (see HealthKnowledge). In the UK these aspects are largely overseen by the UK National Screening Committee, which considers evidence on benefits, harm and risks before recommending that the NHS develops a particular screening programme and is increasingly presenting screening as risk reduction to emphasise this point.

Factors to consider about screening

While screening programmes can bring benefits, they can also have some risks. As part of the criteria noted in the Introduction under ‘When is screening appropriate?’ the following are considered by the UK National Screening Committee before a screening programme is implemented:

1. False negatives – provide false reassurance and can mean that some individuals may not seek medical advice when they should.
2. False positives – result in anxiety and further invasive diagnostic tests or treatment which risk side effects and carry economic cost.
3. Overdiagnosis - for example, the detection of non-lethal lesions that meet the histologic criteria for cancer or cancer precursors and will lead to unnecessary treatment. A special collection issue of the British Medical Journal (from 4th March 2015) (subscription required but accessible, for example, via Athens or the NHS Knowledge Network) focuses on the issue of overdiagnosis, including that resulting from screening (for example papers by Thornton, 2018, Johansson et al., 2015, Van den Bruel et al. 2015, and Baratt, 2015).
4. Harm of the screening test, including inconvenience and potential psychological harm.
5. Epidemiological artefact - may increase the perceived benefits of the screening programme due to:
   • Lead Time Bias: screening appears to increase survival time simply because the disease is detected earlier. Once controlled for, there may be little or no positive impact of the screening test as the total survival time can be the same with or without screening.
   • Length Time Bias: An overestimation of survival because long-duration cases are more likely to be detected and treated than short-duration, and more dangerous, conditions.
6. Detection rate (i.e. the proportion of the total referred for diagnosis which turn out to be actual cases) – if this is reduced by the introduction of a screening programme, i.e. if more people need to be screened to identify one case, this will create pressure on health systems. For a good example see Weatherhead and Lawrence’s (2006) analysis of the impact of the creation of open access melanoma screening clinics where detection rates subsequently reduced.

Only when a screening programme is assessed as having potential to do more good than harm at the population level will it be advocated and policy recommendations made by the UK committee. Best practice for the introduction of screening programmes has been described (Gray et al., 2008), but assessing the long-term impacts can be challenging and the impacts of screening on mortality have been found to be equivocal (Saquib et al., 2015, Prasad et al., 2016).

Current screening programmes in Scotland

Details of the current screening programmes and who they are for in Scotland can be found on NHS Inform. They include screening for:

• Bowel cancer
• Breast cancer
• Cervical human papilloma virus (HPV)
• Abdominal Aortic Aneurysms (AAA) 
• Diabetic retinopathy 
• Pre-school orthoptic vision screening
• Newborn screening 
• Pregnancy screening

For further information see the following, which also contain many public information publications:

• Public Health Scotland screening pages.
• Pregnancy and newborn screening website.
• Scottish Government screening pages.
• National Services Division.
• Scottish Diabetics Retinopathy Screening Collaborative.

Other screening

Other forms of screening are also undertaken in Scotland:

• Blood and tissue donations are automatically tested for various communicable diseases and may undergo some discretionary testing, for example, for malaria, West Nile Virus, Trypanosoma cruizi. Tissue donations are also tested for bacterial and fungal contamination
• Genetic screening.

For further information see:

• Joint United Kingdom (UK) Blood Transfusion and Tissue Transplantation Services Professional Advisory Committee.
• NHS Blood and Transplant testing of tissue and blood.
• Scottish National Blood Transfusion Service.
• NHS Choice genetic testing.

Policy context

The Scottish Government follows the advice of the UK National Screening Committee. This is an expert committee that advises ministers and the NHS in the four UK countries about all aspects of population screening and supports implementation of screening programmes. It considers all aspects of screening including the case for introducing new population screening programmes and for continuing, modifying or withdrawing existing population programmes against a set of internationally recognised criteria. The committee recently published a manual on screening in healthcare aimed at policy makers, public health leaders and screening stakeholders across the health system.

In 2017 the Scottish Government established a Scottish Screening Committee to consider the implementation of all UK National Screening Committee recommendations in the context of the specific Scottish circumstances.