INTRODUCTION: Studies have shown that women with a false-positive result from mammography screening have an excess risk for breast cancer compared with women who only have negative results. We aimed to assess the excess risk of cancer after a false-positive result excluding cases of misclassification, i.e. women who were actually false-negatives instead of false-positives.

METHOD: We used data from the Copenhagen Mammography Screening Programme, Denmark. The study population was the 295 women, out of 4743 recalled women from a total of 58,003 participants, with a false-positive test during the screening period 1991-2005 and who later developed breast cancer. Cancers that developed in the same location as the finding that initially caused the recall was studied in-depth in order to establish whether there had been misclassification.

RESULTS: Seventy-two cases were found to be misclassified. When the women with misclassified tests had been excluded, there was an excess risk of breast cancer of 27% (RR=1.27, 95% confidence interval (CI), 1.11-1.46) among the women with a false-positive test compared to women with only negative tests. Women with a false-positive test determined at assessment had an excess risk of 27%, while false-positives determined at surgery had an excess risk of 30%.

CONCLUSIONS: The results indicate that the increased risk is not explained only by misclassification. The excess risk remains for false-positives determined at assessment as well as at surgery, which favours some biological susceptibility. Further research into the true excess risk of false positives is warranted.