Whenever I order a mammogram for a woman in her 40’s, I also give her a warning: “Don’t get scared if it’s abnormal.” I tell her this because research shows that a woman who undergoes 10 routine screening mammograms has a 50-50 chance of having something unusual that requires her to go for more tests. The vast majority of these mammographic abnormalities aren’t cancer, but she still needs to get the additional tests, just to make sure.

So the new mammogram recommendations by the United States Preventive Services Task Force really didn’t surprise me. While there’s pretty good evidence that mammograms save lives in women age 50 and older, it’s not a great test in younger women. Women under 50 are more likely than older women to have false positive mammograms, resulting in their needing additional testing for something that turns out not to be cancer. They are also far less likely than older women to have breast cancer detected by mammograms.

When you look at the overall population, mammography’s lack of precision in picking up cancer in younger women is pretty astounding. According to data from the Breast Cancer Surveillance Consortium, a network of mammogram registries, 556 women in their 40’s have to get a screening mammogram for the test to pick up one invasive, or potentially life-threatening, cancer. One out of a thousand screened women will have a breast cancer that’s not picked up by their mammogram, while close to 1 out of 10 women in this age group will have a false positive result. For older women, the test is more precise. For example, 200 women in their 60’s have to be screened to find one invasive cancer, and there are fewer false positives.

One recent academic article pointed out that even though the advent of mammography 30 years ago led to a surge in the number of women diagnosed with tiny, localized breast cancers, it hasn’t significantly decreased the number of women found to have disease that’s already spread to other parts of the body. If mammograms were truly effective, the article’s authors argued, there should have been a bigger drop in the number of women with advanced cancer, because their disease should have been caught before it was able to spread. Some researchers contend that many of these tiny cancers, called ductal carcinomas in situ, won’t grow, and, by finding them, widespread mammography has resulted in lots of women being “overtreated” with aggressive therapies.

Despite all these concerns, the fact remains that breast cancer kills 40,000 women in the United States every year, more than any cancer except lung cancer. Given all the questions about mammograms’ effectiveness, we clearly need better ways to screen women. An ideal screening test would pinpoint women who are at high risk of developing an aggressive breast cancer. This would allow doctors to monitor these women more vigilantly, perhaps with more frequent mammograms and other tests, such as ultrasounds, while those at low risk wouldn’t need to be tested as often. It would also help women make a well-informed decision about whether to take medicine to prevent breast cancer.

Such tests could be especially important for black women, who are more likely than whites to develop aggressive cancers at a younger age. Routine mammograms often miss fast-growing cancers, which can pop up during the one or two year interval between screening tests.

Unfortunately, we’re not there yet. True, there are a few tests to identify some women at high risk, such as those who have abnormal changes, or mutations, in genes called BRCA 1 and 2. But these gene changes account for only a small fraction of breast cancers. The National Cancer Institute has a computer tool that uses information about a woman’s personal history to calculate her overall risk, but it only gives a very general estimate. The institute spent more than $36 million last year to fund studies looking at such tests, including one called ductal lavage, which collects cells from inside the breast. But these tests aren’t yet ready for widespread use.

Finally, it’s important to note that the new guidelines don’t say women in their 40’s should avoid screening mammograms. Instead, they recommend that these women talk with their doctors about mammography’s benefits and harms before deciding what’s best for them.

Unfortunately, given the sad state of primary care medicine today, in which doctors spend less and less time talking to patients, these conversations often don’t occur. Too often, the mammogram is a test that’s just ordered with little discussion and not much thought. Hopefully, the new guidelines will spur more conversations between doctors and women about what mammograms can and can’t do – and women will be better prepared to understand their results.

Erin Marcus is an internal medicine physician and writes at New America Media.

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