When it comes to medicine, most of us are uncomfortable with ambiguity. We want definite answers backed by solid evidence. But even when the best research has been done, we often don’t have clear answers.

Certain antibiotics work against certain infections most of the time. Surgery will usually cure certain diseases.

Always, on the other hand, isa rare term in medicine, and it’s certainly not used in reference to screening mammograms. We want screening mammograms to detect all dangerous cancers in their early stages, but yield no ill effect.

That’s not the option available to us. Instead we are told that screening mammograms can reduce death from breast cancer ---at the cost of false positives and overdiagnosis. There is no always, no never. Only ambiguity.

Even the organizations tasked with making recommendations about these things don’t agree. The United States Preventive Services Task Force (USPSTF) is revamping its mammogram recommendations this year. They have not released the final report yet, but the draft gives a grade of C to screening mammograms before age 50 and a grade of B to biennial screening mammograms from age 50 to 74.

The American Cancer Society (ACS) has now released their own revised recommendations, promoting annual mammograms from age 45 to 54, bumping the starting age up from 40. They recommend transitioning to biennial exams after age 54.

Other countries range widely in their policies. The Swiss Medical Board recommends against systematic screening with mammography and instead explains, “Providing clear, unbiased information, promoting appropriate care, and preventing overdiagnosis and overtreatment would be a better choice.” Meanwhile, in Uruguay, women are obliged to undergo mandatory screening mammography every two years from age 40 to age 59.

In the U.K., women aged 50 to 73 receive free mammograms every three years by invitation. The invitations stop for older women, but they can still make their own appointment and receive the screening after age 73. In Germany it’s every two years from age 50 to 69. France is similar, covering mammograms every two years from age 50 to 74.

All of these groups have access to the same data, but they’ve come to radically different conclusions on how it should impact health policy. This is hard to understand until you look at some numbers.

Without screening, a woman has a 2.7 percent lifetime risk of dying from breast cancer. With the updated USPSTF guidelines, a woman will receive 13 screening mammograms during her lifetime and the risk drops to 2.0 percent. The ACS guidelines would have a woman receive 20 screening mammograms and drop the risk to around 1.8-1.9 percent.

So why not go for the 20 mammograms? It’s not a huge drop in mortality, but any drop must be good, right?

Not so fast. Sixty-one percent of 40 or 50-year-old women screened annually for ten years with mammograms will have a false positive screen. This could mean stress, another mammogram, an ultrasound, or even a biopsy for something that isn’t cancer.

And around 19 percent of breast cancers diagnosed with screening mammography are overdiagnosed. Meaning, yes, they are cancers but, no, the cancer would never have caused a problem for the woman. Since it’s impossible with today’s technology to determine which cancer is an overdiagnosis and which is not, they all get treated. And cancer treatment isn’t easy.

Now it becomes a value judgment. At what point are we willing to trade off the harms of false positives and overdiagnosis to save a life?

And while no one wants to say this, I will. All mammograms cost money. Work-ups of false-positives cost money. Treatment of overdiagnosed breast cancer costs money. The bucket of healthcare funds does have a bottom. A dollar spent in one place isn’t available to spend in another place. The financial cost of screening mammography must play into recommendations, as much as we hate that fact.

So now the conflicting recommendations start to make more sense. Each organization and each health care system looks at the entire picture and makes a value judgment.

But who’s to say one is wrong and the other is right?

You are. Unless you want a job in Uruguay, then tough luck. And that’s why the conflicts in recommendations are a positive thing. They bring you, the patient, back into the decision-making loop.

After a discussion of risks and benefits with her doctor, one woman wants to avoid all risk of dying of breast cancer. She’s got important things to do. She opts for screening, early and often.

But another woman comes to a completely different conclusion. She feels the risk of breast cancer is low enough already. She doesn’t want the stress or an open door to unnecessary medical interventions. She doesn’t want to be debilitated in any way while she is living her life. She opts for fewer screening mammograms.

They are both right. This is an important lesson for just about any screening test. They all offer risks and benefits in varying proportions. Blanket statements cannot account for individual values.

So don’t be frustrated when you see the news reports with conflicting recommendations for breast cancer screening, prostate cancer screening, or any other type of screening test. Instead, look at it as the opportunity it is to take charge of your health care.