False Positives Screening Mammography

Mammography and clinical breast examination are the two principal means of screening for breast cancer.1 The effectiveness of breast-cancer screening has been well documented in eight randomized, controlled trials,2 but there has been less attention to its accuracy in community settings and to the consequences of a false positive result. A national review of community mammography facilities in the United States found that 1 of every 10 screening mammograms gave a false positive result.3 Equivalent information for clinical breast examination is not available.

If a woman undergoes annual screening beginning at the age of 40, she will have had 60 opportunities for a false positive result by the age of 70, with 30 mammograms and 30 clinical breast examinations. The cumulative lifetime risk of her having a result from a screening test that requires further workup, even though no breast cancer is present, is not known. An estimate of 25 percent has been given for the cumulative risk of a false positive result after 10 mammograms and 10 clinical breast examinations.4 It is important to determine the cumulative risk of false positive tests, because women are advised to have breast-cancer screening every one to two years over several decades of their lifetimes, and false positive results can provoke anxiety, increase costs, and cause morbidity.5-13

Using the computerized clinical records of a health maintenance organization (HMO) for a group of women over a 10-year period, we determined the cumulative risk of a false positive result of breast-cancer screening, the number and type of subsequent diagnostic workups resulting from the false positive results, and the costs of the false positive results. The HMO we studied has long encouraged women who are 40 or older to undergo routine breast-cancer screening. By studying themedical records, we ascertained the 10-year cumulative rates of false positive results for both mammography and clinical breast examination. We then determined the number of diagnostic examinations generated by the false positive results and estimated their costs.