The women in our study were aware of false positive results from screening mammography but seemed to view them as an acceptable consequence of screening. Although studies attest to the short term physical and psychological impact of false positive results,5–9
our respondents were highly accepting of them: most would not take them into account when deciding about screening, and almost 40% would tolerate 10
000 or more false positives requiring biopsy for each life saved.
One explanation for this high tolerance is that women have an overly optimistic sense of the benefit of mammography. We found no evidence to support this explanation. No respondent thought screening mammography eliminated the chance of dying of breast cancer. Women were aware that mammography misses some cancers (actually underestimating the reported sensitivity). Most women also recognised that health promoting behaviours such as not smoking, exercising regularly, and eating a low fat diet were more beneficial than mammography in prolonging life, which is true for the average 60 year old woman.28
Alternatively, it might be posited that women did not fully understand the consequences of a false positive result (for example, anxiety, pain, inconvenience, or extremely rare harms such as severe infection or death related to anaesthesia). We found, however, that women who had had false mammograms expressed the same high tolerance as women who had not. Thus, women seemed to think that false positives are worth the reassurance of being told they do not have cancer. Similarly, Gram et al found that almost half of women with false positive mammograms viewed the experience as having an overall positive impact on their lives,8
and most women continue to undergo mammography.29
Women's perceptions about a potential diagnosis of ductal carcinoma in situ differed noticeably from their perceptions about false positive mammograms. In contrast to false positives, most women were unaware of non-progressive forms of breast cancer and even doubted their existence. Once informed about non-progressive cancer, the women seemed concerned. Most wanted to take into account the possibility of ductal carcinoma in situ when deciding about screening. Younger women, in whom 90% of the cancers found by screening mammography are ductal carcinoma in situ,15
were the most interested in such information. In addition, women's reported willingness to treat ductal carcinoma in situ increased as we hypothetically increased the chance of ductal carcinoma in situ progressing to invasive breast cancer, suggesting that such information might influence decisions.
One limitation of our study is the representativeness of the sample: we did not include women in households without a telephone and those who requested that their name be removed from the database. This left about 80% of US women eligible for sampling. Secondly, although our sample represents women across a broad range of age, education, and income, it differed from the general population: the women were wealthier and better educated, and almost all were white. Women from ethnic minorities and women with the lowest socioeconomic indicators were underrepresented. Subsequent studies are needed to assess whether such women have different perceptions.
One concern is the possibility of systematic bias in our sample because respondents differed from non-respondents. Our response rate of 66% lessens but does not eliminate this concern. Respondents and non-respondents did not differ by age (the one variable available for comparison). Our main findings were, however, extreme enough (for example, only 7% of women were aware of non-progressive breast cancer) to suggest that even if respondents and non-respondents differed noticeably, our overall conclusions should remain robust. Although the survey was long and complex, response rates for items were high. Response rates were lowest for the question on the treatment threshold for ductal carcinoma in situ (82%) and averaged 98% for all other questions.
Although experts on screening have focused much attention on the anxiety experienced by women with false positive mammograms,13
we believe clinicians counselling women about mammography should spend less time reviewing what most women know and accept—that is, that false positives are part of screening. In contrast, more time should probably be spent educating women about the less familiar outcome of the ambiguity associated with the detection of ductal carcinoma in situ.
What is already known on this topic
False positive results and diagnoses of non-progressive cancer are recognised problems of screening mammography
Little is known about how women feel about these problems
What this study adds
Almost all of the 479 women (99%) knew that false positive mammograms occur
Women do not seem to think that false positive mammograms are an important harm of screening—even women who have had a false positive result
Women's tolerance of false positives is not explained by overly optimistic beliefs about the benefit of mammography
Few of the women (6%) were aware of the possibility of non-progressive cancer