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Testing a Screening Test

In Bandolier 3 the criteria used to test a test were discussed.

When testing a new test against an established diagnostic test or procedure, improvements in sensitivity, specificity and predictive value are sought, as well as cost and patient acceptability. These simple criteria need to be applied to all new screening tests, and to many screening tests presently in use.

Screening - a major problem for the nineties

Screening tests are relatively inexpensive when compared with the treatment of chronic disease or major operations, but the total expense of all screening programmes in the UK is considerable, probably about £500 million per annum.

No screening test should be introduced until it has been subjected to rigorous evaluation and meets strict criteria (1). The criteria used to assess the screening test should be even stricter than those used when considering a new diagnostic test, for screening tests are, by definition, provided for healthy populations. There are quite different ethical contracts between the provider of screening and the healthy person invited to be screened and those which exist between the clinician and the patient who has sought help for a problem that is causing concern.

Screening tests should only be offered to the population after it has been proved that they are effective and that the resources are available to deliver a high quality service to the population as a whole. Only two or three of the twenty or so screening tests on offer to the population meet these strict criteria.

The need for high quality in screening

In a screening test the balance between adverse and beneficial effects is often fine. High quality screening is essential if benefits are to outweigh the social and financial costs of screening, particularly when it is remembered that the social cost of screening - for example, the anxiety that results from a false positive test - is borne by a healthy member of the population who will not benefit from the screening programme as an individual.

The relationship between quality and the balance between costs and benefits is shown in the figure below.

No screening tests should be offered, therefore, unless there are:
  • explicit quality criteria;
  • an information system which collects the data that allows the achievement of standards to be measured;
  • a system for taking managerial action if standards are not being met.

The Chief Medical Officer's national screening initiative

The new Journal of Medical Screening has been published by BMJ Publications, and at a conference to launch this journal the Chief Medical Officer made a major speech outlining the steps that should be taken in future to ensure that only effective screening tests which can be delivered at high quality are offered to the public.

The system proposed is set out in the box below.

Four types of screening test

The various screening tests that exist at present can be classified into four main groups.
  • Tests whose effectiveness is unknown and which are not available to the public.
  • Tests of unknown effectiveness which are currently being offered to the public.
  • Tests of proven effectiveness which need to be introduced in a systematic and controlled way.
  • Tests of proven effectiveness which are being offered to the public but which need quality improvement.

A strategy for tests of unknown effectiveness

The National Research and Development Strategy has identified screening as one of its main targets for action. The Standing Group on Health Technology has set up a Population Screening Panel, chaired by Dr Sheila Adam, Director of Public Health in North West Thames Regional Health Authority. The Population Screening Panel has already identified five tasks which have been classified as being of the highest priority by the Central Research and Development Committee. These are listed below.
  1. Screening for colorectal cancer: to assess whether screening by flexible sigmoidoscopy once at age 55 to 60 can reduce mortality from colorectal cancer.
  2. Screening for stroke by identifying high blood pressure and treating it effectively : to inform NHS decision-makers about areas where policy is clear and areas where policy or further research is needed.
  3. Review of the management of mildly or moderately dyskaryotic cervical smears: to provide clear guidelines and informed policy regarding the effectiveness and relative cost-effectiveness of different referral and management policies for women with mildly or moderately dyskaryotic cervical smears.
  4. Antenatal screening for HIV: to assess the feasibility, costs and benefits of screening pregnant women for HIV in areas of high prevalence.
  5. Evaluation of methods of screening for Down's syndrome: to assess the costs, benefits (from the perspective of purchasers, providers and users) and effectiveness of different methods of screening in order to inform policy.
There are two important points about this list of priorities:
  • it covers both screening tests which are not yet available and existing screening tests;
  • it does not cover all the screening tests which need critical appraisal; in particular the rapid developments in genetics and molecular medicine.

The Chief Medical Officer's initiative

The development of a clear programme of screening research is the first step, and a vitally important one in improving screening in England. The Chief Medical Officer, in his major speech at the screening conference, outlined the steps that needed to be taken to ensure that decisions to implement screening resulted in high quality and effective screening programmes. He cited the Breast Screening Programme as an example of successful screening. A number of factors were identified that led to the success of this programme, notably:
  • an explicit assessment of the technology - the Forrest Report;
  • a clear national policy decision;
  • the identification of resources earmarked for screening;
  • strong national leadership and co-ordination;
  • a policy advisory committee which could monitor the impact of the policy;
  • the Departmental Committee chaired by Professor Martin Vessey;
  • the development of explicit and agreed quality standards which provided a strong basis for quality assurance.
The Chief Medical Officer proposed that the same approach be used to improve the quality of cervical screening. Much has been achieved in the last five years by the activities of the National Co-ordinating Network but more resources are needed for stronger national co-ordination and clearer and more explicit quality standards. He also proposed that a National Screening Network (NSN) be set up.

The National Screening Network

The National Screening Network will seek to link the various screening programmes, both new and existing programmes, to ensure co-ordinated programme development and quality improvement. Dr Muir Gray, a Regional Director of Public Health, has been asked to act as Co-ordinator of the National Screening Network which will be part of the Chief Medical Officer's Public Health Network.

Objectives of the National Screening Network

  1. To ensure that screening programmes are not introduced until a national policy decision has been made by supporting health authorities seeking to stop the piecemeal and unco-ordinated implementation of screening tests as a result of local enthusiasm.
  2. To identify screening tests or programmes that require reappraisal of their effectiveness and appropriateness, submitting information of evidence to the Population Screening Panel of the Standing Group on Health Technology.
  3. To ensure that new screening programmes are introduced in a co-ordinated fashion.
  4. To co-ordinate and support the work of national co-ordinators of individual screening programmes to improve the effectiveness and quality of screening.

Priorities for 1994/95

At the screening conference a number of priorities were identified.
  • The development of criteria that can be used to assess genetic screening programmes.
  • The more tightly co-ordinated implementation of Down's syndrome screening to ensure that those populations covered by Down's syndrome screening are covered by a high quality and effective service.
  • A review of blood pressure screening.
  • A review of child health screening to identify which screening tests need particular consideration and review.

Giving advice to those who purchase health care

The principal objectives of the National Screening Network and the Population Screening Panel will be to give advice to those who commission health services. On the basis of information currently available, it is possible to give clear and categorical advice on certain screening programmes.
  • Screening for ovarian cancer should not be introduced or offered except as part of a controlled trial of its effectiveness.
  • Screening for carcinoma of the prostate, recently highlighted in the House of Commons debate on the early detection of cancer, should not be offered to the public unless as part of a controlled trial.
  • Whole population screening for raised levels of cholesterol should not be offered to the population, although about £40 million or £50 million of medication is currently being prescribed to reduce cholesterol levels as a result of haphazard and unco-ordinated cholesterol testing.
  • Screening for abdominal aortic aneurysm should not yet be introduced as a standard screening programme.
The work of the National Screening Network will be featured in future editions of Bandolier.
J.A. Muir Gray, MD, FRCP(Glas), MRCGP, FFCM,
Director of Health Policy and Public Health.

Reference:

JMG Wilson & G Jungner. Principles and Practice of Screening for Disease. WHO 1966.


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