This may encompass a range of activities whose purpose is:
to reduce the chance of a person contracting a disease or becoming disabled.
to identify either an increased susceptibility to develop a disease, or an early sign of a disease at a stage which will still allow treatment to be effective.
It can be divided as follows:
which aims at the complete avoidance of a disease (for example, by immunising a child against an infectious disease – see IMMUNISATION).
which aims at detecting and curing a disease at an early stage before it has caused any symptoms. This requires ‘screening’ procedures to detect either the early pre-symptomatic condition, or a risk factor which may lead to it. An example of the former is cervical cytology, where a sample of cells taken from the cervix of the UTERUS and examined microscopically for abnormality. An example of the latter is CHOLESTEROL measurement as part of assessing an individual's risk of developing ischaemic heart disease (see under HEART, DISEASES OF). If it is significantly raised, dietary or drug treatment can be advised.
aims at minimising the consequences for a person who already has the disease (e.g. advising people to take more exercise and stop smoking after a heart attack).
The range and extent of opportunities for prevention are expanding as research identifies the causes of diseases and more effective treatment becomes feasible. Inevitably there is economic and political debate about the cost-effectiveness of prevention versus cure, as well as about the ETHICS.
Screening tests might be simple questions (e.g. ‘Do you smoke cigarettes?’ – this predicts a considerable increase in the risk of chronic bronchitis, heart disease, bronchial cancer and many other diseases, and enables targeted advice and help to stop smoking to be given). Other screening tests involve carrying out complex special investigations such as blood tests or the microscopic investigations of cells – for example, for precancerous changes.
Many conditions can be identified at an early stage before they cause symptoms or signs of disease and in time for effective treatment to be carried out. Inevitably, some of the screening tests proposed can be expensive (particularly if used in large populations), painful or inaccurate and may not improve the results of treatment. Screening can also provoke considerable anxiety in those waiting for tests or results. Therefore, over the years considerable research has been carried out into the appropriateness and ethics of screening, and the World Health Organisation in 1968 identified a set of rules for evaluating screening tests:
The condition sought should be an important health problem, for which there should be an accepted treatment for patients with recognised disease.
Facilities for diagnosis and treatment should be available if a case is found.
The screening test or examination must be suitable and valid. A false positive test may cause massive anxiety and also considerable expense in proving that there is no disease. Similarly, false negatives can lead people to be reassured and to ignore serious symptoms until too late. If large numbers of positive tests or false positives occur during a screening programme, health services can be swamped.
The test, and any treatment as a possible result, should be acceptable. For example, there is little point in screening for a fetal abnormality which, if found, would lead to a recommendation for termination if the mother will refuse it on religious or moral grounds.
Screening tests also need to be considered from an economic perspective and the cost of case-finding (including diagnosis and treatment of patients diagnosed) balanced in relation to possible expenditure on medical care as a whole.
Finally the programme should reflect the natural history of the disease, and case-finding should normally be a continuing process and not a ‘once for all’ project.
If these rules are followed, considerable benefits can result from well-planned and well-managed screening programmes, and they form an important part of any health-care system. The extent to which manipulation of genetic material will be added to more traditional approaches such as counselling, immunisation and drug treatment cannot yet be predicted but, as time goes by, it is often likely to be ethical and social controls which limit developments rather than technical and scientific limits.