Deciding what to cover and what not to cover is health care’s costliest question.  With its ruling last week, the Supreme Court showed that America doesn’t have as much time as it thought to get the answer right. By deciding that human genes cannot be patented, the Court did more than open the door to a rush of newer, more affordable forms of genetic testing for cancer and other diseases.  It provided a glimpse at the kinds of breakthroughs that will define medicine’s next decade, enabling people to receive diagnoses long before they show symptoms. But a near-constant string of genetic breakthroughs will soon bring a never-ending challenge to the question: Who pays?

To guide our decisions, the debate should focus on a simple question: What is the value of knowing? 

Not all things are worth knowing.  Some procedures have been found to be unnecessary or harmful.  For instance, CT scans should not automatically be done on a child with a minor head injury; simple observation can be as good and spares radiation.  A routine chest X-ray is unnecessary prior to outpatient surgery for patients with an unremarkable history and physical exam; they rarely improve patient outcomes. To identify the proper course of treatment, clinicians have developed criteria to assist them in prescribing appropriate tests and therapies. One example is the American Board of Internal Medicine Foundation's Choosing Wisely campaign, which lists 130 such recommendations. Guidelines like these could be a starting point to right-size the system.

On the other hand, some things are worth knowing. In Angelina Jolie’s case, the value of knowing her genotype (she had a damaged BRCA1 gene) and her phenotype (her mom died of ovarian cancer at 56) after a predictive genetic test led to a diagnosis that she had an 87 percent chance of breast cancer, empowering her to undergo a preventive double mastectomy, reducing her cancer risk to five percent. The Affordable Care Act requires that private insurers cover the costs of genetic counseling and testing for similar high-risk women, without co-payment.

But how should we apply the value of knowing to other breakthroughs? Consider lung cancer.  In 2010, the National Cancer Institute found that long-time smokers who underwent advanced CT screening had a 20 percent lower death rate from cancer than those who had standard chest X-rays, since it allowed patients to make changes early.  Yet, CT scans are not covered.  For a disease that takes 160,000 American lives each year, should they be?

Or consider thyroid cancer.  According to a recent study, a new genetic test accurately diagnoses benign growths on the thyroid gland, much more consistently than the current test, one in four of which are inconclusive, leading to many unnecessary surgeries.  For a disease that affects 60,000 Americans each year, should the more expensive genetic test be covered, too?

Sometimes, the value of knowing doesn’t lead to cures, but can improve patient well-being, as with Alzheimer’s disease. Studies show that older Americans who receive early Alzheimer’s diagnoses can actively participate in decisions about their future, while better avoiding accidents and falls. A new imaging test can pinpoint the presence of a plaque linked with Alzheimer’s. Not every person with this plaque has Alzheimer’s, but its absence can allow it to be ruled out, potentially leading to other treatable causes.  With Alzheimer’s cases expected to triple by 2050, should this test be covered?

Our system will never be able to pay for every new treatment and procedure that comes available.  But we need to get smarter about how we choose.  For interventions, health experts use a metric called quality adjusted life years, which determines the cost of year per life saved. A drug that costs more than $100,000 per year of life saved is a harder sell than interventions like colon cancer screenings, which are in the $20,000 range.  There is not yet an equivalent paradigm for diagnosis, something a robust debate over the value of knowing could address.

For years, predictive genetic testing has tantalized us with the possibility of knowing more about ourselves, through our DNA. The Supreme Court just opened a new door to that genetic future. It’s up to us to decide how often—and to what extent—the value of knowing is worth the value of paying for it.  

Sorensen is a former professor of radiology at Harvard Medical School. He is the U.S. chief executive officer of Siemens Healthcare.