False-positive screening mammograms and biopsies among women participating in a Canadian provincial breast screening program.

Background: Mammography screening results in false positives that cause anxiety and utilize scarce medical resources for their resolution. Determination of screening recommendations requires knowledge of the population risk of false positives.

Methods: Data were extracted from the Screening Mammography Program of British Columbia and analyzed to determine the influence of personal factors including age, ethnic group and screening history, and the centre where screening was performed, on the likelihood a new screen would result in a false positive and whether a biopsy was required. The resulting probabilities were combined to provide values for lifetime screening algorithms.

Results: Age, screen sequence number, history of previous abnormal screens and centre where screening was performed were significantly related to the likelihood a new screen would be a false positive. British Columbia women screened biennially between the ages of 50 and 69 have a projected 41% chance of a false-positive screen and a 5.6% risk of a related biopsy, with the best performing centres having rates of 26% and 3%, respectively.

Interpretation: Model projections for BC overall are comparable to other North American estimates. Estimates varied depending upon screening centre attended.