The Cascade of Screening

Screening is not a single test, but rather a cascade of events that can result in either benefit or harm (Figure 12.2). The first step is the screening test itself. Although some diagnostic tests can have useful intermediate results, with a screening test the result is either positive or negative. If a recommendation for anything other than continued routine screening comes from the screening test, it is a positive test. The patient is notified that all is not well and that further evaluation of some kind is needed.

Typically, with cancer screening, many more people have a negative screening test than a positive. After a positive test, further workup is required to determine whether the screening test is a true positive or a false positive. The workup may vary depending on the degree of positivity of the screening test or other circumstances. Some people with false-positive tests have ongoing anxiety related to the experience of screening whereas others do not.

People with a true-positive test do not all benefit from earlier detection of their cancer. These people fall into four categories. Category 1 includes people with fast-growing, malignant disease for which treatment is ineffective. These people do not benefit from earlier detection. Category 2 includes people whose cancer is easily treated regardless of when it is detected. These people also do not benefit from earlier detection. Category 3 includes people whose cancer would never have caused important clinical problems and does not need to be treated. These people have "pseudodis-ease" and do not benefit from earlier detection. Category 4 includes people whose cancer is more effectively treated earlier, after screening detection, than later, after clinical detection. These are the people who benefit from earlier detection. Treatment of people from category 3 is overtreat-ment; any adverse effects from treatment of this group must be counted as among the harms of screening. This situation happens frequently because category 3 cancers cannot always be accurately distinguished from other categories.

We next consider two categories of potential harms from screening: false-positive screening tests and overtreatment. Although false-negative tests could also theoretically cause harm by false reassurance, we know of little evidence to substantiate this potential harm.

10 Ways To Fight Off Cancer

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