A false positive result from a screening mammogram often leads to emotional, physical, and economic stress, though false positives are an expected part of proper medical care. For this study, the authors obtained data for women who received mammography screening through a large health care organization with multiple facilities in the greater metropolitan Chicago area. Among the 741,150 screening mammograms from 261,767 women included in the analysis were 12.3 percent that yielded a false positive result; the remaining 87.7 percent yielded true negative results.
Writing in Cancer Epidemiology, Biomarkers&Prevention, they found that women who had a false positive result were significantly less likely to have a subsequent screening mammogram in the database than women who had a true negative result; 22.1 percent of women who had a true negative and 15.0 percent of women who had a false positive had at least one subsequent screening mammogram in the database.
Among the women with more than one screening mammogram in the database, those who had a true negative result from the first mammogram were 36 percent more likely to have returned for a subsequent screen in the next 36 months compared to women who had a false positive result.
When delay in having a subsequent screen was defined as any mammogram performed more than 12 months after the first screening mammogram, the median delay was found to be significantly higher for women who had a false positive result compared with those who had a true negative result. The median delay was 13 months for those who had a false positive result compared with three months for those who had a true negative result.
The researchers then reanalyzed the data using different statistical methods to eliminate the chance for bias in the methods. Using propensity scoring matching, they found that women who had a true negative result were 34 percent more likely to return for a subsequent screen than those who had a false positive result and that the median delays in returning for a subsequent screen were six months and 13 months, respectively.
In addition, the researchers found that the four-year cumulative risk of a late stage at diagnosis was 0.4 percent for those who had a false positive screening mammogram compared with 0.3 percent for those who had a true negative result. This difference was statistically significant.
The main limitation of the study is the lack of ability to follow the women who did not show up for their next screen in this health care system because some may have received subsequent mammograms at different institutions, though the results were similar when restricted to those who remained in the system and so the researchers are confident that this limitation did not bias the results.