Up to one third of women given a mammogram could suffer the anguish of a false positive where the test incorrectly detects breast cancer and consequently further unnecessary medical intervention, including biopsies and ultrasound scans, say researchers.

The risk of a false positive result reached as high as 64 per cent after 10 mammograms, they discovered. Clinical breast examinations did not fare much better, with the risk of a false positive result after 10 inspections being 22.3 per cent.

In a US survey of 2,400 women aged between 40 and 69 who had a total of 9,762 mammograms and 10,905 clinical examinations the estimated cumulative risk of a false positive from either procedure was nearly half, at 49.1 per cent.

The false positive results led to 870 outpatient appointments, 539 diagnostic mammograms, 186 ultrasound examinations, 188 biopsies and one hospital stay. For every $100 spent on screening, a further $33 was spent evaluating the false positive results, estimate researchers from the University of Washington School of Medicine in Seattle (New Eng J Med, 1998; 338: 1089-96).