What is screening?
Screening in the 21st century means a screening programme which is aimed at the risk reduction for the screened individual, is based on sound evidence, delivered to a pre-agreed policy and standards and is quality assured.
It is the testing of apparently well people to find those at increased risk of having a disease or disorder. Those identified as being at risk are sometimes then offered a subsequent diagnostic test or procedure. Screening differs from traditional clinical use of tests where patients feel unwell and seek consultation, diagnosis or treatment. It is also different to case finding - i.e. where you would look for additional illnesses in those with medical problems. Screening test is not a diagnostic test. Some people use the concept 'sieve' in relation to screening - so you're sieving through an apparently healthy population to find the people at risk of a condition or disease.
The aims of screening are:
- To test people who either do not have or have not recognized the signs or symptoms of the condition being tested for
- To give information about risk that is deemed valuable for that individual even though risk cannot be altered
- To reduce risk for the individual of future ill-health in relation to the condition/disease being tested for.
What is the purpose of screening?
Screening can improve health and can bring about long lasting benefits. For example, cervical screening brings about early detection of cervical cancer and thereby reduce the number of women who suffer severe consequences from cervical cancer. In new-borns, the bloodspot screening programme can identify early on Phenylketonuria (PKU), Congenital hypothyroidism (CHT), sickle cell disorders, cystic fibrosis (CF) and Medium Chain Acyl Coenzyme A Dehydrogenase Deficiency (MCADD) and thus help babies to get the treatment they require as soon as possible and prevent any disabilities that may have arisen. In short, screening can help increase life expectancy and quality of life.
An important factor to note is that while screening programmes can help, they all have the potential to harm. Screening is done in seemingly healthy individuals who are not seeking medical help and may want to be left alone and this can deem the screening programmes unacceptable to a number of people. There can also be a demand for screening programmes that can lead people to implement expensive programmes of no clear value (hence why screening programmes are only implemented in accordance to certain criteria, see below). Screening can be unpleasant, inconvenient, costly, stigmatize and can lead to individuals perceiving that they are unwell when they are not. For example, cervical screening detects abnormal cells not cancerous cells - a diagnostic test is required to determine that - however people can mistake a negative screen for a diagnosis. Another example is in relation to Down's Syndrome Screening - again a screen does not mean that the child has Down's Syndrome or not, it only detects whether or not the child could be at risk of it.
Pros of Screening
1. Many diseases - e.g. breast cancer -have a genuine lead time where case fatality is lower in populations offered the test (not just a better outcome in those identified early).
2. It brings more benefit than harm when based on sound evidence relating to programme outcomes and when it is delivered as a quality assured programme.
Cons of Screening
1. False positives can swamp true positives - unnecessary anxiety, increased demands for investigations.
2. Need to weigh up the seriousness of disease and look at issues of cost, invasiveness, acceptability and whether a better test exists.
3. If small numbers are used, smaller samples have wider confidence intervals. This leads to a greater degree of uncertainty in PPV (and imprecision of specificity).
4. Screening can lead to harmful treatment.
Positives and Negatives
True positive is someone who actually has the disease and was also positive on the test.
False positive is someone who was positive on the test (i.e. found to be unhealthy) but who actually does not have the disease/condition (so it is an incorrect result).
True negative is someone who does not have the disease/condition and who was also negative on the test.
False negative is someone who is found to be negative on the test (i.e. healthy) but who actually has the disease/condition.
Ideally you want a test that will accurately detect true values and minimize the number of false results.
It is impossible to have a screening programme without false positives and false negatives.
Sensitivity & Specificity
Sensitivity (detection rate) is the ability of a test to find those with a disease or condition. Specificity is the ability of a test to identify those without the disease or condition. Both sensitivity and specificity are important to public health in setting up screening programmes. High specificity means that you would have no/or few people wrongly labelled as diseased (false positives). High sensitivity would have no/or few missed cases (false negatives). It is rarely possible to have a test that is 100% sensitive and 100% specific. A compromise level has to be reached.
Getting the balance right between the two is important as you want to be able to discriminate between those with and those without the disease. Where the cut-off or trade off is depends on the implications of the test (is the aim to exclude those without disease or detect those with so that you can start prompt intervention?) Sensitivity below 100% means that some people with the disease will be missed and specificity below 100% means that some healthy people will be told that they have a disease/condition, causing anxiety. The consequences will depend on the disease/condition concerned. Typically, people use ROC (a curve which shows the relationship between sensitivity and specificity). The nearer the ROC is to the left, the better. It can also be used to compare different screening tests. People also use likelihood ratio - i.e. sensitivity/1-specificity. This provides an odds of a positive test occurring in someone with the disease as opposed to someone without.
Test (or Screen)
Positive Predictive Value (PPV) = a/a+b; Negative Predictive Value (NPV) = d/c+d
Present Not present
Clinicians are more interested in predictive values than sensitivity and specificity. A positive predictive value (PPV)is the likelihood of someone having a disease being detected.
Negative predictive value is the likelihood of someone who doesn't have the disease being correctly predicted as such. When interpreting predictive values, clinicians should be aware of the approximate prevalence of the condition or disease of interest in the population being tested.
When prevalence is high, predictive value of a positive test is high. For example, 2% or a rate of 2 per 100 is a common disease and any disease with this prevalence or higher will have larger samples giving greater precision. If prevalence is low, you will end up screening loads of people before you can yield any true positives. So even if you have an excellent test (e.g. with over 98% sensitivity and specificity), you will have poor predictive positive values in low prevalence settings. This could mean people being incorrectly labelled, suffer anxiety or being referred to further diagnostic tests when there is no need.
What is a Screening Programme?
Before the 1950s, the only routine screening available in the UK was medical and dental checks for children. Since then, there has been a growing interest in identifying conditions and diseases early on in order to have early intervention or treatment and with that was the concern that screening be provided as part of a quality assured programme delivery. Today screening has become a programme not a test. It encompasses the whole system or programme of events from identifying the eligible population to delivering interventions and supporting individuals who suffer adverse effects.
A screening programme needs to be adequately planned to ensure that the population receive a high quality, acceptable, reliable and effective service. No screening programme will be implemented unless there is robust evidence and that there can be comprehensive quality-assurances of the programme. Given, the balance of harm and good associated with screening, it is important to look at how many people need to be screened to find one case or prevent one death (numbers needed to screen), how many people would be adversely affected by screening per life saved and whether or not the proposed screening test would meet the criteria of a screening programme.
A screening programme works as follows:
- the eligible population is invited for a screening test
- those whose test results are positive are asked to have a further diagnostic test
- anyone who is found to have a positive diagnostic test is then referred onto the intervention stage.
It is important to note that while you can test positive in the screening phase it doesn't mean that you have the disease or condition.
The decision to introduce a screening programme occurs at a national level and will involve a national body who will appraise the evidence before making a recommendation to the government. In the UK, this is done by the UK National Screening Committee. If a programme is agreed, then work commences on a national level on deciding the aims and policies, the procurement of equipment, workforce planning, information management and communications strategy. In England, NHS England & Improvement commission screening programmes as part of Section 7a public health commissioning. They commission the provision and monitor the performance of the programmes. Providers are responsible for identifying and inviting eligible people, taking and reading of tests, scheduling follow-up and running failsafe systems, providing information, support and advice for participants and producing performance and monitoring statistics. All screening programmes are integral to patient care pathways.
Once set up, screening programmes are consistently quality assured to ensure that high quality programmes are being delivered. Quality assurance entails explicit standards, an information system that collects data to allow each programme to be compared with the standards and a management system that allows action to be taken if a programme fails below acceptable standards. In the England, this is done by the PHE Quality Assurance Screening Team. Information on what they do can found here.
Before a screening programme is introduced, it must comply to strict criteria. In the past, the Wilson and Jungar criteria was used but in the UK, this has since been surpassed by the UK National Screening Criteria.
Types of Screening Programmes
- Antenatal screening programmes
- Newborn screening programmes
- Cancer screening programmes (e.g. breast, bowl and cervical)
- Adult and young people screening programmes (e.g. AAA, diabetic retinopathy)
This is the testing for inherited or heritable disorders in people without signs or symptoms or known genetic susceptibility. The same principles of screening apply here. An additional feature is that genetic testing can yield information that affects other family members who did not themselves have a test or give informed consent to the information being uncovered.
Grimes at al. "Uses and abuses of screening tests". The Lancet, 2002: 359:881-84.
Raffle & Gray. Screening: Evidence & Practice, Oxford University Press: 2007.