What is Screening?
Although screening programmes have now been in place for many years in the UK and other countries around the world, there is still confusion as to what they can and cannot achieve. A screening test for a disorder is one offered to a large group of people who have no symptoms of the disorder. They are usually selected on the basis of their age or sex or the fact that they are pregnant. The screening test will pick out the people who are at highest risk of having the disorder in question. It cannot usually divide for certain those who have the disorder from those who don't have it. For most tests, some people with a positive result will turn out not to have the disorder ('false positive') and some with a negative result will subsequently be shown to have the disorder ('false negative'). Those people who have a positive result on the screening test are usually offered a further test ('diagnostic test'), which will pick out much more accurately those who have the disorder.
The term 'screening programme' includes the screening test, the diagnostic test and any treatment or action that follows on from these. To be of value, the quality of life of the person being screened, or their family, should be improved as a result of some change that results from the screening programme. This could be the early treatment of a disease so its adverse effects are minimised, or it may be allowing a pregnant woman to choose whether she wishes to continue with a pregnancy when the unborn baby has a serious disease. This latter is referred to as 'informed choice' and the success of such a programme cannot be judged by how many babies with a disease are terminated. It must be evaluated on the basis of the proportion of women who feel that they have received enough information and support to allow them to proceed in the way that is best for them. The benefits of the programme should outweigh any hazards, of which there are always some, such as anxiety generated.
Because screening, apart from the neonatal bloodspot programme, was somewhat haphazard, the National Screening Committee (NSC) was set up in 1996 to advise the UK Health Ministers on screening policy. In its first report, the NSC defined a screening test as:
'The systematic application of a test or enquiry, to identify individuals at sufficient risk to benefit from further investigation or direct preventive action, amongst persons who have not sought medical attention on account of symptoms of that disorder.'
The NSC is responsible for providing advice on those screening programmes which are to be implemented, those which are to be discontinued and those which are to be amended. Individual screening programmes are assessed against a set of criteria covering the condition, the test, the treatment options and effectiveness and acceptability of the screening programme. Assessment in this way is intended to ensure that more good than harm is achieved at a reasonable cost.
Appropriate information must be offered at every stage of the screening programme and where an abnormal test result occurs, counselling should be available.
The NSC has a subgroup known as the Fetal, Maternal and Child Health Coordinating Group. It is responsible for looking at issues in pregnancy and childhood.
Antenatal Screening
Antenatal screening is undertaken for a number of conditions in the mother and unborn baby. For conditions in the mother, for example anaemia and raised blood pressure, treatment is available during the antenatal period to improve her health and indirectly that of the baby. For conditions in the baby, the screening test may enable a mother to make an informed choice about continuation of the pregnancy or allow treatment to improve the baby's health to be started as soon as possible.
Screening for maternal disease
Screening for maternal disease involves monitoring pregnancies to try to select those where, even though the pregnant woman appears well, she has a condition such as anaemia, HIV infection, syphilis or high blood pressure which could be harmful to the unborn baby. Once the condition has been discovered, treatment can be started to lessen the effect of the disease on the unborn baby. Immunity to rubella is checked, so that any non-immune mothers can be vaccinated after delivery. If a mother is found to be infected with hepatitis B, then her baby can be immunised soon after birth to prevent him/her becoming infected.
Screening for abnormalities of the unborn baby
Screening for fetal abnormality is offered to pregnant women as part of routine maternity care. Screening falls into two main groups - those that make use of ultrasound examination and those that rely on a blood test.
Ultrasound examination has the potential to show a number of structural abnormalities of the unborn baby. Abnormalities of the heart, spine, face and brain can be picked up as can neural tube defects (see entry, Spina Bifida). Early detection allows a mother to make an informed choice about the continuation of her pregnancy and, in some cases, treatment can be planned to take place soon after the baby is delivered.
Blood tests and/or ultrasound scans may indicate that the unborn baby has a high risk of being affected by Down syndrome. If this is the case then a further 'diagnostic' test either chorionic villus sampling or amniocentesis (see section on Pre-Natal Diagnostic Techniques in Patterns of Inheritance) depending on how far the pregnancy has progressed - may confirm the presence or absence of the condition in the unborn baby. A blood test on the mother may also show that the unborn baby might have a disorder of its blood such as sickle cell disorder, thalassaemia or haemolytic disease (Hemolytic disease - US) of the newborn (a form of severe anaemia in the baby arising because it has a different blood group from its mother). In this case, however, further tests would need to be performed to confirm this.
The results of antenatal blood tests are only capable of providing an estimate of the risk of a fetal abnormality and cannot definitely confirm the presence or absence of a specific condition during pregnancy. As such, screening test results are reported as a probability or risk of an affected pregnancy. These are described as:
- screen positive results - where the pregnancy is identified as having a high chance of a fetal abnormality.
- screen negative results - where the pregnancy is identified as having a low chance of a fetal abnormality.
However, there are a number of limitations of screening:
- Firstly, the tests currently available are capable of detecting a limited number of fetal abnormalities only. Many fetal abnormalities are not identified by the tests currently offered to women during pregnancy. This means that every woman has a small chance of having a pregnancy affected by a fetal abnormality even if they have had all the screening tests. For example, the screening tests cannot pick up whether a child will have autism or psychiatric problems.
- Secondly, the results of screening tests are not 100 per cent accurate. The reason for this, as previously indicated, is that screening tests cannot completely distinguish those pregnancies affected with particular fetal abnormalities from those which are not.
Therefore:
- For some pregnancies shown to have a high chance of fetal abnormality (positive screen), the unborn baby will not have the condition screened for. These are called "false positive results". In fact, for certain conditions, the majority of women found to be in the high risk group have a normal diagnostic test and go on to have healthy babies.
- Conversely, for some pregnancies shown to have a low chance of fetal abnormality (negative screen), the baby will be affected by the condition. The number or proportion of these so-called "false negative results" varies with the type of screening test. A negative screen result does not mean that an affected pregnancy has been completely excluded.
In fact, the accuracy of many screening tests depends on knowing how far advanced a pregnancy is at the time the test is performed. This is why all women have an ultrasound examination early in pregnancy.
- Thirdly, while screening tests incur no physical harm to the mother, some diagnostic tests carry a small risk of miscarriage. Following a positive screen result, therefore, women who decide to undergo diagnostic testing, such as chorionic villus sampling, amniocentesis or blood sampling from the fetus, may lose the unborn baby as a result of the procedure. This is one reason why invasive diagnostic procedures are not offered to every pregnant mother as a screening test to check for the presence of a fetal abnormality.
- Fourthly, screening cannot give complete reassurance to women and may generate anxiety.
Antenatal Counselling
Informed Decision-Making
The National Screening Committee (NSC) has proposed that the key objective of screening programmes is to enable people to make informed choices. Individuals have a limited time during pregnancy in which to make decisions about screening and diagnostic testing. Typically, the decisions they face include:
- Whether or not to be screened
- Which conditions to be screened for
- Which test(s) to have
Women whose pregnancies are found to be at risk (that is a positive screen result) would need to consider:
- Whether or not to undergo prenatal diagnostic testing
- Which test(s) to have
- If the baby is found to be affected whether or not to continue with the pregnancy
Antenatal screening and diagnostic tests differ significantly from tests employed in most branches of medicine. The majority of diagnostic tests in medicine are intended to guide management so that treatment may be implemented to alleviate or cure a medical condition. However, for the majority of fetal conditions which may be detected by antenatal screening, there may be no treatment currently available. In these cases the only options available for the parents are either to continue with the pregnancy and to use the test result to "prepare themselves" for the birth of an affected child, or to accept the option of undergoing a termination of pregnancy. The very existence of screening tests will therefore mean that many parents will face the decision of whether or not to continue with their pregnancy. As there is likely to be a continuing rapid growth in genetic science, leading to the introduction of new screening tests, it is possible that many more couples will be faced with difficult dilemmas arising from screening and diagnostic testing concerning termination for fetal disease or abnormality.
Information needs
The provision of up-to-date information and high quality counselling and support enables women and couples to make the choices which they consider best. In this regard, The Royal College of Obstetricians and Gynaecologists recommend that:
- Women and their partners should feel free to exercise the options of their choice.
- Screening and diagnostic tests must only be undertaken with the knowledge and consent of the individual woman.
- For all aspects of antenatal screening, women and their partners must be given verbal information supported by suitable written or audiotape/audio-visual, if required.
- For all aspects of antenatal screening, women and their partners should be aware of the risks and benefits associated with each test.
- It is suggested that there should be a policy on how, and when, women and their partners are given results. Results of specific tests such as those for fetal abnormality or HIV should be given in person.
- Individuals should be informed of all the results of any tests undertaken.
- Women should be fully supported and be offered access to specialist advice and support if required. This may include counselling or specialist advice for genetic disorders, HIV or haemoglobinopathy etc. It should also include referral to self-help groups including ARC - Antenatal Results and Choices (see entry, Fetal Abnormality)
- Women should be satisfied with the service offered. Adherence to these policies can be audited.
Antenatal support
In order to minimise the emotional distress often associated with screening programmes, individuals may be offered accurate and sensitive counselling at various stages including:
- Prior to screening, during which individuals are given the opportunity to make an informed choice about whether or not to have particular tests.
- After screening, during which individuals are given the results of investigations and presented with the options for future action. At this time, support is offered to individuals in decision-making.
- Post-decision making, during which information from subsequent investigations is given, support is offered concerning the decision about whether or not to continue with an affected pregnancy and the implications for the future are discussed.
- Prior to a subsequent pregnancy, in which parents are helped to make decisions about antenatal diagnostic tests and cope with the huge burden of anxiety which often accompanies subsequent pregnancies (even if all goes well).
Quality assurance and monitoring
Quality assurance and monitoring are essential components of any screening programme and it is one of the NSC's responsibilities to set national standards. Currently, service provision across the four UK countries is fragmented leading to variation in policy and practice in antenatal screening. To illustrate this, different screening tests are offered in different regions, some at the same stage of pregnancy and some at different stages. Some screening tests indicate the risk of fetal abnormality at term whilst others express this risk at the time of screening. Variation in screening policies has resulted in the provision of screening on an inequitable basis and an overall line of responsibility would help to reduce such inequality.
Looking to the future, resources made available through the NHS plan have allowed the setting up of an Antenatal Screening Programme Development Project. This consists of a small national team with regional co-ordinators.
By improving the overall quality and co-ordination of antenatal screening programmes, it is hoped that women will be offered better information and support to enable them to make the choices they consider best.
Neonatal Screening and Childhood Screening
Neonatal screening
Neonatal screening is offered so that the presence of a congenital disorder in the newborn may be identified as soon as possible after birth and treatment offered. Neonatal screening aims to ameliorate disabling conditions that impair a child's quality of life. The timeliness of screening ensures that appropriate treatment may begin and lead to the maximum possible reduction of the adverse effects of the condition.
Screening takes a number of forms in the neonatal period:
- A very small quantity of blood is taken from the baby's heel between the fifth and eighth day after birth. Currently all babies are tested for Phenylketonuria and congenital hypothyroidism (see entry, Thyroid Disorders) cystic fibrosis and sickle cell disorders. A pilot of screening for MCADD (Medium chain acyl-coA dehydrogenase deficiency), another inborn error of metabolism (see entry, Metabolic diseases), been completed and screening will be introduced nationally by spring 2009.
- All babies have a routine physical examination after birth. A general examination ensures that there are no visible malformations and more specifically, the eyes are examined for conditions such as cataracts, the heart for heart defects, the hips for developmental dyspasia (see entry, Congenital Dislocation and Developmental Dysplasia of the Hip) and the testes for cryptorchidism.
- All babies have their hearing tested in the first four to five weeks of life.
Childhood screening after the neonatal period
- All babies have a physical examination at 6-8 weeks. This takes the same form as that in the neonatal period.
- It is recommended that all children have their vision assessed between 4 and 5 years old by someone specially trained to do so. This will take some time to institute.
- All children should have their height, weight and hearing screened at school entry.
Quality assurance and monitoring
There is variation in practice and policy in neonatal and childhood screening. There is currently a structure of child health screening co-ordinators in England who work closely with the antenatal co-ordinators.
Conclusion
Screening has significant differences from clinical practice as the health service is targeting apparently healthy people and offering to help individuals to make better informed choices about their health. However, as the NSC has pointed out, there are risks involved in screening and it is important, therefore, that individuals have realistic expectations of what a screening programme can deliver. Although screening may have the potential to save lives or improve quality of life through the early diagnosis of serious conditions, it is not a foolproof process. As such, whilst screening may reduce the risk of developing a condition or its complications, it cannot offer a guarantee of protection. The NSC has indicated that in any screening programme, there is an irreducible minimum of false positive results (people wrongly reported as having the condition) and false negative results (people wrongly reported as not having the condition). The NSC is increasingly presenting screening as risk reduction because of this. The NSC believe that what is required is overall direction, a written policy, specified funding and line responsibility, at the same time, preserving local commitment. It will be interesting to see the extent to which service providers will encourage public awareness of screening in the future.
Glossary
Screening test
This is a test which is designed to identify those individuals who are at a high enough risk of having a particular disorder to warrant the offer of a diagnostic test. A screening test may be a procedure, such as a blood test, or it may be the asking of a question, such as "How old are you?"
Screening programme
This includes screening, diagnosis and the management of a condition.
Positive result (on a screening test)
This is a result which indicates that an individual is at high risk of a condition.
Negative result (on a screening test)
This is a result which indicates that an individual is at low risk of a condition.
False positive result
A positive result is indicated where the condition is not present.
False negative result
A negative result is indicated where the condition is present.
Sensitivity of a screening test
This refers to the ability of the test to accurately detect those who have a condition. A highly sensitive test has a sensitivity approaching 100 per cent. The consequence of a test which lacks sensitivity is that individuals are informed that they do not have a condition when in fact they do (false negative).
Specificity of a screening test
The ability of the test to accurately identify those who do not have the condition. A highly specific test has a specificity approaching 100 per cent. The consequence of a test which lacks specificity is that an individual is informed that they have a condition when in fact they do not (false positive).
Positive predictive value
The proportion of people with a positive test result who have the condition.
Further information on the NSC and the screening programmes is available at Web: http://www.library.nhs.uk/screening
Additional Reading
Collins, J. and Dezateux C. (2001) The UK National Newborn Screening Programme Centre: Working towards quality in partnership. Great Ormond Street Hospital for Children NHS Trust
Elliman, D.A.C., Dezateux, C., Bedford, H.E. Newborn and childhood screening programmes: criteria, evidence, and current policy. Arch Dis Child 2002;87:6-9.
Health Departments of the United Kingdom (1998) First Report of the UK National Screening Committee
Health Departments of the United Kingdom (2000) Second Report of the UK National Screening Committee
Royal College of Obstetricians & Gynaecologists (1995) Report of the Audit Committee's Working Group on Communication Standards, Royal College of Obstetricians & Gynaecologists, London.
Royal College of Obstetricians and Gynaecologists http://www.rcog.org.uk has a range of useful information in its Guidelines section.
Medical text written November 2002 by Contact a Family. Approved November 2002 by Dr David Elliman. Last updated November 2007 by Dr David Elliman, Consultant in Community Child Health, Islington Primary Care Trust, London, UK and Great Ormond Street Hospital, London, UK and member of the National Screening Committee and Dr Helen Bedford, Senior Lecturer in Children's Health, Institute of Child Health, London, UK and past member of the Sub-Committee on Child Health, National Screening Committee.
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