Screening

Screening is 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.¹ The National Screening Committee criteria for appraising the viability, effectiveness and appropriateness of a screening programme¹ are based on the criteria developed by Wilson in 1968 and address the condition, the test, the treatment and the screening programme. The National Electronic Library for Health (see Internet and Further Reading below) has full details of the current screening programmes in the UK.

• Knowledge of disease:
  • The condition should be important.
  • There must be a recognisable latent or early symptomatic stage.
  • Natural course of condition, including development from latent to declared disease, should be adequately understood.
• Knowledge of test:
  • Suitable test or examination.
  • Test acceptable to population.
  • Case finding should be continuous (not just a "once and for all" project).
• Treatment for disease:
  • Accepted treatment for patients with recognised disease.
  • Facilities for diagnosis and treatment available.
  • Agreed policy concerning whom to treat as patients.
• Cost considerations:
  • Costs of case finding (including diagnosis and treatment of patients diagnosed) economically balanced in relation to possible expenditures on medical care as a whole.

The condition

• The condition should be an important health problem.
• The epidemiology and natural history of the condition, including development from latent to declared disease, should be adequately understood and there should be a detectable risk factor, disease marker, latent period or early symptomatic stage.
• All the cost-effective primary prevention interventions should have been implemented as far as practicable.
• If the carriers of a mutation are identified as a result of screening, the natural history of people with this status should be understood, including the psychological implications.

The test

• There should be a simple, safe, precise and validated screening test.
• The distribution of test values in the target population should be known and a suitable cut-off level defined and agreed.
• The test should be acceptable to the population.
• There should be an agreed policy on the further diagnostic investigation of individuals with a positive test result and on the choices available to those individuals.
• If the test is for mutations the criteria used to select the subset of mutations to be covered by screening, if all possible mutations are not being tested, should be clearly set out.

The treatment

• There should be an effective treatment or intervention for patients identified through early detection, with evidence of early treatment leading to better outcomes than late treatment. There should be agreed evidence-based policies covering which individuals should be offered treatment and the appropriate treatment to be offered.
• Clinical management of the condition and patient outcomes should be optimised in all healthcare providers prior to participation in a screening programme.

• There should be evidence from high-quality randomised controlled trials that the screening programme is effective in reducing mortality or morbidity. Where screening is aimed solely at providing information to allow the person being screened to make an informed choice (e.g. Down's syndrome, cystic fibrosis carrier screening), there must be evidence from high quality trials that the test accurately measures risk. The information that is provided about the test and its outcome must be of value and readily understood by the individual being screened.
• There should be evidence that the complete screening programme (test, diagnostic procedures, treatment/intervention) is clinically, socially and ethically acceptable to health professionals and the public.
• The benefit from the screening programme should outweigh the physical and psychological harm (caused by the test, diagnostic procedures and treatment).
• The opportunity cost of the screening programme (including testing, diagnosis and treatment, administration, training and quality assurance) ) should be economically balanced in relation to expenditure on medical care as a whole (i.e. value for money).
• There should be a plan for managing and monitoring the screening programme and an agreed set of quality assurance standards.
• Adequate staffing and facilities for testing, diagnosis, treatment and programme management should be available prior to the commencement of the screening programme.
• All other options for managing the condition should have been considered (e.g. improving treatment, providing other services), to ensure that no more cost-effective intervention could be introduced or current interventions increased within the resources available.
• Evidence-based information, explaining the consequences of testing, investigation and treatment, should be made available to potential participants to assist them in making an informed choice.
• Public pressure for widening the eligibility criteria for reducing the screening interval, and for increasing the sensitivity of the testing process, should be anticipated. Decisions about these parameters should be scientifically justifiable to the public. If screening is for a mutation the programme should be acceptable to people identified as carriers and to other family members.

• Screening can reduce the risk of developing a condition or its complications but it cannot offer a guarantee of protection.
• In any screening programme, there is an irreducible minimum of false positive and false negative results.
• Screening is therefore increasingly presented as risk reduction.

Although screening programmes may benefit populations, not all participants will benefit and some will even be harmed by participation.³

• Personal costs include problems with false positive results, which can lead to distress and possible unnecessary treatment.
• Even individuals who choose not to participate in screening may be disadvantaged, for example being labelled as from a "positive family" with regard to genetic susceptibility, when other family members have chosen to be screened and found positive..
• False negatives also can occur, as no test is 100% sensitive, which can then lead to false reassurance by both patients and doctors. This may even dissuade patients from returning for future screening tests.
• Misinterpretation of results can lead to a false sense of security, e.g. patients with normal cholesterol or normal blood pressure may continue to smoke.
• Costs to society: Actual costs of equipment, services, treatment etc; also, the time taken off work for people to attend for the screening test and the treatment.
• Psychological costs involved. One article showed that false positive results in screening tests can have undesirable effects.⁴ Women who had been given the "all clear" after having had abnormal mammography results three years previously remained significantly more anxious than those who had normal results. This was actually sufficient to deter 15% of them from attending for mammography at all the next time round.
• Prophylactic mastectomy although perhaps an effective intervention in BRCA mutation, requires evidence on psychological and social impact.
• Some people have different health beliefs and cultures and object to being screened. This needs to be appreciated when considering individual autonomy.
• Implementing screening tests may mean that funds are diverted away from other services, e.g. cancer treatments.