creening is defined as testing for a condition when the person has no recognized signs or symptoms of that condition. The purpose of screening is not to merely detect a condition, but rather to help people live better or longer. This is an important distinction: the detection of earlier disease by itself is insufficient to justify a screening program. The program must additionally demonstrate that people live longer or better because of the earlier detection.
A positive screening test result does not indicate that a person has the condition, but rather that he or she has a higher probability of having the condition. People with positive screening tests usually undergo diagnostic testing to determine whether the condition is present. For example, a woman with a positive mammogram result does not necessarily have breast cancer, but she may undergo a needle localization biopsy to determine whether she has breast cancer.
Screening is not a single test, but rather a cascade of events that can lead to either benefits or harms (see The Cascade of Screening, later in this chapter). Potential benefits include living better or living longer and are usually experienced some years after screening. Potential harms include the effects of false-positive or false-negative screening tests and problems that result from overdiagnosis and overtreatment. Harms are usually suffered soon after screening.
Because screening programs may lead to either net benefit or net harm, decision makers must carefully evaluate proposed programs. Eight criteria distinguish effective programs:
1. Disease: The disease should cause a sufficient burden of suffering to warrant attention and should have a detectable preclinical phase of sufficient length to allow early detection (see The Critical Point in Cancer Treatment, later in this chapter).
2. Test: The screening test should be sufficiently sensitive to detect those cancers that could benefit from earlier treatment. Note that the test does not need to be maximally sensitive but rather "sensitive enough" to detect those cancers that it is important to detect. Cancers that are important to detect are those which are treatable when detected by screening but not when detected clinically.
3. Test: There are usually many more false-positive test results than true-positive results. The screening test should be specific enough to minimize the number of false-positive test results so as to minimize their negative consequences.
4. Availability and acceptability: The screening test, workup, and resultant treatment should be available to all and acceptable both to clinicians and to the people being screened.
5. Treatment: There must be a treatment for the disease that is more effective when applied to screening-detected cancers than clinically detected cancers. By "more effective," we mean that people will live longer or better as a result of this earlier treatment.
6. Harms of overtreatment: Often earlier detection includes detection of people with intermediate lesions that would never progress to invasive cancer. Screening may lead many people with these lesions to be subjected to treatment that they do not need and which causes harm. To minimize harms, people with lesions that will not progress to clinically important disease should rarely be subjected to potentially harmful and unnecessary treatment.
7. Benefits and harms: Overall benefits (in terms of people living longer or better) must outweigh overall harms (including harms from the screening test, harms from the workup, the adverse effects of earlier treatment and overtreatment, the psychologic effects of labeling, and the downstream effects of surveillance).
8. Costs: The net health benefits must come at a reasonable cost.
Screening for cancer is a popular idea, but this popularity may be based more on intuition than on understanding. Studies show that the great majority of Americans are convinced that being screened is part of being a responsible citizen.1 What is less certain is how well the public comprehends the magnitude of the potential benefits of cancer screening; even less certain is whether the public appreciates the magnitude of the potential harms. Further, one could wonder whether the public has a reasonable grasp of the gaps in our knowledge of the effects of screening.
One might argue that whether the public understands these issues is irrelevant. The fact is that the public has decided that cancer screening is a good that it desires. We suggest that there are several important reasons for the public to better understand screening. The first is that screening consumes resources, such as money and the time of medical personnel. In a system strapped for resources to appropriately care for all our people, expending resources on services that offer little benefit and risk greater harm reduces the contribution medical care can make to the health of the public. The second reason is that if widespread screening fails to reduce the rate at which people die of cancer, ultimately the public will ask why it does not. If the medical care system has not informed the public of the limitations of cancer screening as a strategy to reduce cancer mortality, its credibility will be damaged. Finally, if the public pins all its hopes for cancer control on screening, this attitude may inhibit creative new ideas and research that could develop alternative strategies for cancer control.
The public needs to have a clear idea of both sides of the cancer screening coin: benefits and harms. Clinicians must play a large role in this educational effort. This chapter attempts to help clinicians better understand these issues so that they can appropriately advise the public.
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When over eighty years of age, the poet Bryant said that he had added more than ten years to his life by taking a simple exercise while dressing in the morning. Those who knew Bryant and the facts of his life never doubted the truth of this statement.