Obese women who have mammograms are much more likely to receive abnormal readings from the tests than are women of ordinary weight, a new study reports.

An obese woman is 20 percent more likely to be told that she has a positive result and that she needs more tests, resulting in anxiety for her and extra health-care costs, the researchers said. The study, led by Joann Elmore of the Washington University Harborview Medical Center in Seattle, appears in the current issue of The Archives of Internal Medicine.

The findings are based on a review of more than 100,000 mammograms performed in the Northwest United States. The women studied were divided into three groups - normal weight or below, overweight and obese - based on their body-mass index, determined by height and weight. The results were surprising, the researchers said, because obese women are more likely to have fatty breast tissue, which is easier to read in mammography than is dense tissue. The study also found that women of ordinary weight were more likely to get false positives than were underweight women.

In general, obese women are at higher risk than other women for breast cancer after reaching menopause. When cancer is found, it is often at a more advanced stage, perhaps because it is harder for the women to detect small growths during self-examination.

Ellen Schur of Harborview said doctors should explore whether other imaging tests might be better at detecting cancer in obese women.