Getting a positive mammogram result, then learning on follow-up that nothing is really wrong, is good news. But a false positive mammogram has a dangerous downside: It may reduce the likelihood that a woman returns for later screenings, increasing her risk for a late-stage cancer.

Researchers looked at 90,918 false positive and 650,232 true negative mammograms from 261,767 women.

After controlling for age, ethnicity, family history of cancer and other factors, they found that women with a true negative screen were 36 percent more likely to return within three years for another screening than those who had a false positive. When women with false positives did return, they delayed an average of nine months longer than those with true negatives. The study is in Cancer Epidemiology, Biomarkers & Prevention.

Over four years of follow-up, women who had a false positive were significantly more likely to be given diagnoses of late-stage malignancy. The longer the delay in returning to screening, the greater the risk, though the absolute risk remained small.

“Getting a false positive can be a traumatic experience,” said the lead author, Firas M. Dabbous, an epidemiologist with the Advocate Lutheran General Hospital in Park Ridge, Ill. “But it should not deter women from coming back. Mammography screening is the most common widely accepted tool for detecting early tumors. Early detection results in improved survival.”