Screening Flashcards
Purpose of screening
Screening is most often used to select those people who are at higher risk of developing a disease and to offer them a health intervention aimed at prevention by one of two means:
1) Prevention of serious outcomes of existing disease (secondary prevention) example: screening for breast cancer followed by confirmation of diagnosis and early surgical treatment of those with mammograms suggestive of breast cancer.
2) Prevention of the development of a disease (primary prevention) example: screening for high blood cholesterol levels to select people at higher risk of coronary heart disease for health promotion or cholesterol-lowering drug treatment
3) Selection of people fit enough for a job; example: routine health checks for army recruits;
4) Containment of infection; example: screening new nurses or teachers for tuberculosis or food handlers for salmonella
Primary screening
Prevention of the development of a disease (primary prevention) example: screening for high blood cholesterol levels to select people at higher risk of coronary heart disease for health promotion or cholesterol-lowering drug treatment.
Secondary screening
Prevention of serious outcomes of existing disease (secondary prevention) example: screening for breast cancer followed by confirmation of diagnosis and early surgical treatment of those with mammograms suggestive of breast cancer.
Who gets screened?
Screening can either involve the whole population (mass screening) or selected groups who are anticipated to have an increased prevalence of the condition for which screening has been instituted (targeted screening). An example of mass screening would be to measure the blood pressure of all adults in a population. Measuring blood cholesterol in relatives of people with familial hyperlipidaemia is an example of targeted screening.
How do we organise screening?
Screening can be organised in a systematic way (for example: a list is kept of all the females in a population and each woman is invited routinely for a first mammography test the week after her fiftieth birthday) or it can be organised opportunistically (for example: a general practitioner gets into the habit of taking and recording the blood pressure of every patient, regardless of the reason for the consultation).
Risks for the individual:
there may be health risk attached to the screening tests (for example: exposure to x- rays or the risk of miscarriage after amniocentesis);
there may be health risk attached to further confirmatory tests for those with a positive screening result;
a false positive test may cause unnecessary anxiety;
there may be other unwanted and unplanned effects of positive test (for example, life
insurance premiums may be increased);
a subject with a false negative test may be reassured inappropriately and fail to recognise subsequent warning signs of the disease;
a true positive test may increase anxiety and pose a risk to mental health or quality of life
Over diagnosis
Some screen-detected cancers might never have progressed to become symptomatic in the absence of screening, and some people who have cancer detected by screening would die from another cause before the cancer became evident. These cancers cannot be distinguished from other cancers and will thus be treated. This adverse consequence (harm) of screening is called over-diagnosis or over-detection, and is defined as the detection of cancers that would never have been found were it not for the screening test.
Benefits for the individual
Early detection
Reduced morbidity
Reduced chance of dying for diseased Reassurance for those with normal results
Risks for the society
the opportunity cost of the resources put into screening. For example health care providers can be allocated to screen cigarette smokers for lung cancer. The time and resources devoted to lung cancer screening could have been allocated to other health- promotion activities, some of which could have a greater impact on population health and possibly at lower cost;
the costs of confirmatory tests and of treatment. For example if all adults are screened for raised serum cholesterol levels (a known risk factor for heart disease), then there will be a large increase in the number of people prescribed lipid lowering drugs. Yet, only a minority of these adults taking lipid lowering medication would otherwise have developed heart disease.
Benefits for society
Fewer premature years of life lost Economic benefit from these years Reduced cost of treating advanced disease Less transmission (i.e. of infections
Wilson and junger who guidelines
- The condition being screened for should be an important health problem
- The natural history should be well understood
- There should be a detectable early stage
- Treatment at an early stage should be of more benefit than at a later stage
- There should be a suitable test for the early stage
- The test should be acceptable
- Intervals for repeating the test should be determined
- There should be adequate health service provision for the extra clinical workload resulting from the screen
- The risks, both physical and psychological, should be less than the benefits
- The costs should be balanced against benefits
Determining whether a particular screening programme is of value in a particular community will depend on four main issues:
- feasibility
- effectiveness
- cost
Feasibility
Feasibility will depend on how easy it is to organise the population to attend for screening, whether the screening test will be acceptable (having one’s mouth checked for dental caries is acceptable to most people, but having a sigmoidoscopy to detect colon cancer is much less acceptable), whether facilities exist to carry out more extensive diagnostic tests on those who are found positive at screening (for example, can the hospitals cope with the expected increase in demand for breast biopsies following a mammography programme), and whether there are sufficient resources available to treat everyone confirmed as positive (for example, can the health services afford to provide cholesterol-lowering drugs to everyone found to have an elevated serum cholesterol level).
Effectiveness
Effectiveness is evaluated by the extent to which instituting a screening programme affects health outcomes (i.e. mortality or morbidity). Effectiveness is challenging to measure - particularly when using an observational study design - because of a number of biases:
- selection bias
- lead time bias
- length time bias
Selection bias
People who participate in screening programmes often differ from those who do not. Selection bias can work both ways; people who are at high risk may be more likely to come forward (for example, women whose mothers had breast cancer are at higher risk of breast cancer, and are more likely to request screening) or people at lower risk might be more likely to participate (for example, many women at low risk of cervical cancer have a high risk perception, and are more likely request screening).
