Exploring the inaccuracy of mammograms and the threat of false positives

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Image: Torin Halsey/Wichita Falls Times Record News/AP

In the spring of 2014, Dominique Berry went to see her doctor to have an irritating skin condition examined. During the consultation at the clinic in downtown Chicago, Dominique’s physician noticed a small lump under the skin on her chest.

Most patients who undergo mammograms perceive them to be a fairly accurate way of screening for breast cancer. Indeed, for people who have breast cancer, the test will pick this up roughly nine times out of ten. For people who don’t have the disease, the results of the test will tell you this correctly nine out of ten times. Knowing these statistics and having received a positive mammography result, even before the biopsy Dominique considered it likely that she had cancer. However, a simple mathematical argument demonstrates that the opposite is true.

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Of the 1,032 women to receive a positive result, only 36 of them will actually have breast cancer. To put it another way, if your mammogram comes back positive, the overwhelming likelihood is still that you don’t have breast cancer. Despite appearing to be quite an accurate test, the low prevalence of the undiagnosed disease in the population makes it extremely imprecise.

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