False Positive Test Results

False-positive screening test results can cause harm both psychologically and by adverse effects from unnecessary workups. Although a positive screening test does not mean that one has the disease in question, it does mean a person has been placed into a higher risk group than previously. That is, the risk of having breast cancer is higher among women with a positive mammogram than among women who have not yet had a mammogram. The individual may experience the uncertainty of not knowing whether she has breast cancer. This occurrence usually causes stress to the individual involved; any delay in the diagnostic workup adds to the person's concern.

The psychologic trauma from a false-positive screening test can be increased by incomplete resolution of the situa tion. For example, women with an abnormal mammogram are sometimes asked to return for follow-up mammograms every 6 months (rather than annually). Similarly, some men with a high prostate-specific antigen (PSA) value and a negative prostate biopsy are asked to return in 3 to 6 months for a second set of biopsies because the cancer may have been missed in the first set. People who have had benign colonic polyps removed are sometimes asked to have repeat colonoscopy at more frequent intervals. Some of these people may suffer psychologic stress as a result of prolonging the experience of uncertainty.

Some people who have had seemingly complete resolution of the false-positive screening test (e.g., a woman with an abnormal mammogram who had a negative biopsy and was told she does not have cancer) still have lasting concerns. A study of women after having a false-positive mammogram found that many still had lingering doubts that interfered with sleep or function 6 months after a negative biopsy.10 A recent study found a similar result among men with a high PSA screening test and a negative biopsy for prostate cancer.11

Because false-positive tests lead to workups without clinical benefit, any complication from the workup of a false-positive screening test (e.g., colonic perforation from a false-positive fecal occult blood test) is also a harm from screening. Most workups for positive screening tests will be negative.

Although these psychologic effects and complications from workups may seem of little consequence when compared with the potential for extending life by screening, the weighing of these effects on a population level must take into account that the actual number of false-positive screening tests is far larger than the number of true-positive tests, and larger still then the number of true-positive tests that lead to extended life.

The rate at which a screening test yields false positives is determined by its specificity. Many screening tests have specificities above 90%. Although this sounds very high, specificity in the 90% range guarantees a large number of false-positive tests. This is because specificity is the percentage of all people without the cancer who are classified correctly as having a negative test; 1-specificity is the percentage of people without disease who are incorrectly classified as having cancer (i.e., false positive). In a screening program, however, the number of people without cancer is very large; thus, even 10% (or even 5%) of a large number is still a large number.

In most cases, the number of false-positive screening tests outnumbers true-positive tests by a factor of from 4:1 (e.g., prostate cancer) to 10:1 (e.g., breast cancer) or higher. If we consider the proportion of people who have at least one false-positive screening test over a period of years of repeated screening, the ratio of false-positive to true-positive tests is even larger. In one study, nearly 50% of women had at least one abnormal mammogram over 10 years of annual

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