DENVER—There was substantial public outcry last year when new recommendations for mammograms came out suggesting that women could wait until age 50 to start breast cancer screening—and then only get screened every other year. Figures in support of the new policy were bandied about in the news and in doctors’ offices, regarding lives saved from breast cancer (eight women per 1,000) and false positive results (2,250 per 1,000 women). But in spite of the new recommendations, many women and clinicians were unsure about forgoing the additional screenings called for under the old policy if there was hope of saving even one life.

The U.S. Preventive Services Task Force, which made the recommendations (that women ages 50 to 69 get mammograms every other year), based their decision on analyses of eight randomized controlled trials. Although eight might not sound like a lot of studies for a disease that affects so many women (about one in eight will be diagnosed with breast cancer at some point in her lifetime), when compared with the larger literature on disease screening, “that is an embarrassment of randomized controlled trials,” Ned Calonge, of the Colorado Department of Public Health and Environment, said here Tuesday at the American Public Health Association annual meeting. And by embarrassment, he means “a plethora of data,” he explained.

The one in eight statistic can be confusing, Calonge said, because most women who get breast cancer do not die from it. In fact, from a baseline of 1,000 women who never have a single mammogram, 30 will die from breast cancer (if followed from age 40 onward). And that, Calonge pointed out, is already a small number to try to whittle down.

If screening is bumped from biannual to annual for women starting at age 40, it would save almost one additional breast cancer death per 1,000 women, he noted.

The reason that more screening earlier does not translate into many more lives saved, which has been borne out through studies and meta-analyses, is that “as you get older, the test itself gets better,” Calonge said. The shift in breast composition results in fewer false positives as women reach their 60s, which is “when you really need to push screening—because it becomes a very good test at that age.”

And with more tests comes more potential for harm caused by stress from false positive and unnecessary invasive procedures. Just doubling the number of mammograms performed, Calonge noted, would also double the number of false positives and unnecessary biopsies. (One thousand women getting screened every year between the ages of 40 and 69 will produce some 2,250 false positive results and 158 unnecessary biopsies over those 30,000 mammograms.)

The bottom line, he noted, is what so many public health experts, physicians and journalists offered last November: Women in their 40s “should weigh the risks and benefits” and talk with their doctors about whether they want to start screening for breast cancer early, Calonge said.