False-positive Results Are Common with Cancer Screening
The risk of obtaining a false-positive result from screening for prostate, lung, colorectal, and ovarian cancer is high and becomes cumulatively higher with ongoing screening—after 14 screening tests, the cumulative risk of a false-positive is 60.4% for men and 48.8% for women, according to the results of a study published in the Annals of Family Medicine.
Cancer screening has become an important component of preventive care because cancer is most treatable when caught in the early stages of development. In many instances, screening has been shown to reduce mortality from cancer; for example, regular screening with Pap smear has significantly reduced the death rate from cervical cancer in the United States. However, it is unclear whether screening for other types of cancer reduces the mortality rates, as evidence indicates that some slow-growing cancers are being overdiagnosed; in such cases patients would often die of other causes before the cancer started causing symptoms. As a result, there is some controversy over the frequency and interval of cancer screening.
The Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial is a randomized, controlled trial designed to evaluate the effects of prostate, lung, colorectal, and ovarian cancer screening on disease-specific mortality. The study included 68,436 patients aged 55 to 74 who were randomized to receive screening or usual care. Participants received up to 14 screening tests over the course of three years. For women, the tests included vaginal ultrasounds, chest X-rays, sigmoidoscopies to examine the colon, and measurement of an ovarian cancer marker called CA-125. Men underwent chest X-rays, digital rectal examination, sigmoidoscopy, and measurement of a prostate cancer marker called PSA.
After four screening tests, the cumulative risk of a false-positive result was 36.7% for men and 26.2% for women. After 14 tests, the cumulative risk for a false-positive result jumped to 60.4% for men and 48.8% for women. Furthermore, the cumulative risk of undergoing an unnecessary invasive biopsy procedure based on the results of a false-positive screening test was 28.5% for men and 22.1% for women.
The high rate of false-positives does not mean that all screening is “bad”; however, it indicates one of the risks of consistent, long-term screening. It is important to understand both the risks and benefits of screening and to make informed choices about preventive care.
 Croswell JM, Kramer BS, Kreimer AR, et al. Cumulative incidence of false-positive results in repeated, multimodal cancer screening. Annals of Family Medicine. 2009; 7: 212-222.
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