Screening is the presumptive identification of unrecognized disease or defects by means of tests, examinations, or other procedures that can be applied rapidly.
In advocating screening programmes as part of early detection of cancer, it is important for national cancer control programmes to avoid imposing the “high technology” of the developed world on countries that lack the infrastructure and resources to use the technology appropriately or to achieve adequate coverage of the population. The success of screening depends on having sufficient numbers of personnel to perform the screening tests and on the availability of facilities that can undertake subsequent diagnosis, treatment, and follow-up.
A number of factors should be taken into account when the adoption of any screening technique is being considered:
- Sensitivity: the effectiveness of a test in detecting a cancer in those who have the disease;
- Specificity: the extent to which a test gives negative results in those that are free of the disease;
- Positive predictive value: the extent to which subjects have the disease in those that give a positive test result;
- Negative predictive value: the extent to which subjects are free of the disease in those that give a negative test result;
- Acceptability: the extent to which those for whom the test is designed agree to be tested.
A screening test aims to be sure that as few as possible with the disease get through undetected (high sensitivity) and as few as possible without the disease are subject to further diagnostic tests (high specificity). Given high sensitivity and specificity, the likelihood that a positive screening test will give a correct result (positive predictive value) strongly depends on the prevalence of the disease within the population. If the prevalence of the disease is very low, even the best screening test will not be an effective public health programme.
Policies on early cancer detection will differ markedly between countries. An industrialized country may conduct screening programmes for cervical and breast cancer. Such programmes are not, however, recommended in the least developed countries in which there is a low prevalence of cancer and a weak health care infrastructure. Further, only organized screening programmes are likely to be fully successful as a means of reaching a high proportion of the at-risk population.
Countries that favour cancer detection remaining part of routine medical practice, or that simply encourage people to seek specific tests at regular intervals, are unlikely to realize the full potential of screening.
The success of screening programmes depends on a number of fundamental principles:
- The target disease should be a common form of cancer, with high associated morbidity or mortality;
- Effective treatment, capable of reducing morbidity and mortality, should be available;
- Test procedures should be acceptable, safe, and relatively inexpensive.
In a national cancer control programme, screening programmes should be organized to ensure that a large proportion of the target group is screened and that those individuals in whom abnormalities are observed receive appropriate diagnosis and therapy. Agreement needs to be reached on guidelines to be applied in the national cancer control programme concerning:
- The frequency of screening and ages at which screening should be performed;
- Quality control systems for the screening tests;
- Defined mechanisms for referral and treatment of abnormalities;
- An information system that can: -send out invitations for initial screening; – recall individuals for repeat screening; – follow those with identified abnormalities; – monitor and evaluate the programme.
For a number of reasons, patients often fail to adhere to recommended cancer screening activities. While in many cases both the patients and the health care providers understand the concept of early detection, they fail to comply with recommendations. Non-compliance is a general health problem and one that should be addressed in a comprehensive manner to improve outcome and reduce the waste of resources.
Screening that concentrates solely on a high-risk group is rarely justified, as identified risk groups usually represent only a small proportion of the cancer burden in a country. In planning the coverage of screening programmes, however, steps must be taken to ensure that all those at high risk are included. This requirement may be difficult to fulfil. In screening for cancer of the cervix, for example, those at high risk are often difficult to recruit into screening.
SOURCE: World Health Organization (WHO).