Like cancer vaccines, cancer blood panels aren’t some far-flung, futuristic goal. Multi-cancer early detection tests, known as MCEDs, are a thing—right now. Today. Google the phrase and you’ll find multiple products available for purchase. You can have one at your door in mere days.

It’s a concept that has promise, experts say. “The technology is advancing truly at a rapid pace,” , tells Fortune. “I think all of us in the oncology world think that the next frontier is liquid biopsy”—another term for MCED.

Roughly 20 companies are working to develop a blood test that detects a multitude of cancers in a variety of ways—some by detecting associated proteins, others by looking at alterations in DNA, Dr. Ernest Hawk, head of the Cancer Prevention and Population Sciences division at the University of Texas’s MD Anderson Cancer Center, tells Fortune.

The U.S. National Cancer Institute “is now investing public funds into this space,” Hawk says. “You’re no longer just promoting a company and their interests when you talk about this.”

The trouble—for now—is that researchers aren’t quite sure what to do with the data produced by these tests. It’s a reality that could render them a novelty—albeit a (hopefully) temporary one.

Says Knudsen: “Just because you can detect cancer through an MCED test doesn’t mean it’s time to take action.”

Defining ‘meaningful cancer’

The questions about MCEDs currently vexing researchers, according to Knudsen: 

• Do the tests really measure cancer?
• Is there a clinical benefit to these tests?
• At what point does a medical provider take action on the results?
• How does a doctor counsel someone who has a positive MCED test but doesn’t have signs of cancer on imaging like MRIs or X-rays?

If Knudsen took an MCED test today and her results looked positive for breast cancer, doctors wouldn’t know what clinical guidance to give her, she says. That’s because cancer cells are often destroyed by the immune system and, thus, never morph into meaningful illness.

“The immune system is very good at eliminating early stage cancer,” Knudsen says. “Cancer cells are you, gone bad, and the immune system is trained to identify things that look different from you and attack. It’s something your body does regularly.”

Because of this, “cancer DNA is shed into the circulatory system” on a regular basis, she says: “Just because you find cancer DNA doesn’t mean you have meaningful cancer.”

Doctors could recommend more frequent mammograms in case her immune system fails to take care of the problem, but each would entail additional radiation exposure—and repeat radiation exposure can increase the risk of developing breast cancer.

Screening for ‘the unscreenable’

Clinical trials are ongoing to determine if MCEDs can reliably detect cancers early enough to act, extending quality and duration of life for patients—without causing more harm than good.

The answer to that question may differ by cancer type, Knudsen says—because, in actuality, what we lump together as “cancer” is a collection of more than 200 different diseases.

Researchers may find that MCEDs can detect certain types of cancers early, but not others, “and that’s okay,” she says. “But we have to do the clinical testing to make that assessment.”

In the U.S., about 30% of cancer deaths are due to screenable cancers like breast, prostate, cervical, lung, and colorectal. If a person has a positive MCED test for one of these cancers, but no other signs of malignancy, should they be referred for more frequent screening? It’s one question researchers are currently looking into.

Perhaps the bigger question: what to do for those who have an MCED indicating they have a non-screenable cancer—like pancreatic or ovarian—which account for 70% of all cancer deaths in the U.S. Non-screenable cancers typically don’t present until someone has advanced disease and, thus, less-than-optimal chances of survival. 

“Do we have the option now to screen for the unscreenable?” Knudsen rhetorically asks.

An answer of yes would make MCEDs a true medical miracle. The trouble again, however, would be knowing when and how to act on such information.

What doctors could do in such a case, Knudsen says, is recommend that their patients modify their lifestyle to reduce or eliminate cancer-promoting behaviors like smoking, drinking, and living a sedentary lifestyle, to reduce their overall cancer risk.

The impact of such actions isn’t trivial or theoretical, Knudsen maintains, as more than 40% of cancers are associated with modifiable risk factors.

A prostate cancer blood test as a model

As with cancer vaccines, many of the earliest applications may come for patients who have already been diagnosed with cancer. Those who have been treated for cancer and appear cancer-free on imaging may eventually appear cancer-positive on MCED.

Some MCEDs are “already approved for the identification of cancer recurrence,” eliminating the need for a biopsy, Hawk says.

Prostate cancer could serve as a model for what to do with MCED-positive patients, both those who’ve been diagnosed with cancer and those who haven’t, according to Knudsen. It’s unique in that a blood test has long been available for it—PSA, or prostate-specific antigen, a protein produced by cells in the prostate gland. 

A high level of PSA often signals prostate cancer—and because of this, PSA blood tests are used to monitor men with prostate cancer after surgery or radiation, to see if their cancer has returned.

If a patient’s PSA rises, “we have a biomarker of cancer returning,” she says. “We can’t see the cancer, but we know it’s somewhere.” 

Options, which vary by patient, may include hormone-based therapy, radiation, or simply watching and waiting. But the blood test “gives us time to plan and gives us options,” she says. 

“It’s not a DNA-based assessment, but it’s an example of how a blood draw can help you follow a cancer.”

Tips for those paying out of pocket for an MCED

Though MCEDs for the general public are not yet approved by the U.S. Food and Drug Administration, it’s possible to pay out of pocket for one—they generally cost just under $1,000—and to discuss the results with your doctor. But such tests may not be a great move for those who are under age 55, or who aren’t at a high risk of cancer due to factors like family history, as the majority of cancer diagnoses occur in these populations.

Even if the test seems like a good fit for you, experts warn that the results may be overwhelming, underwhelming, or even anxiety-inducing, at this early stage.

“The patient is going to look to the physician and say, ‘What does this mean?’ And the answer is, we don’t have complete guidance on this yet,” Knudsen says. “Not because there’s anything wrong with the test. We just don’t have enough experience with it to know what is the right thing to tell someone.”

If you do receive an MCED-positive test result, it doesn’t mean you must or even should take action. In such a case, Knudsen recommends that patients “have a meaningful discussion with their clinician” about what the results do mean, and what they don’t mean.

The good news: The kind of conversation you might have with your provider after a positive MCED result is the kind of conversation you can have—and should be having—right now anyway.

Question Knudsen recommends everyone asks their health care provider:

• What is my cancer risk?
• What are the behaviors I can change that could potentially lower my cancer risk?
• What should I know about my familial cancer risk? How should I interpret what I do know?

A person may not have undergone genetic health risk testing, but perhaps they’re aware that their mom did and came back BRCA2 positive, signaling an increased risk of breast cancer. Such information is meaningful and absolutely should be shared with health care providers, experts say.

They caution, however, against relying on genetic health risk information provided by some commercial products like familial DNA tests. While some such products offer clinically actionable information, some don’t. Genetic health risk information provided by ancestry tests may provide a false sense of security by informing users that they don’t have certain concerning gene mutations when they actually do.

Knudsen recommends discussing any genetic health risk data you have access to with a genetic counselor. The list of concerning mutations is constantly growing, she cautions—a mutation scientists aren’t sure about today could be deemed concerning down the road. So, keeping in touch with such a counselor is advisable.

A test that’s too advanced?

For now, it’s too early for medical providers to crutch on liquid biopsy—but Knudsen hopes they can in the near future.

“Right now, it’s a developing technology we’re watching,” she says. “We’re highly supportive of this test being integrated once it’s been shown to have clinical benefit and is FDA approved.”

In the future, Hawk foresees MCEDs augmenting existing cancer screening tests like mammograms and colonoscopies, and serving as a screening for cancers we currently can’t screen for.

“They’re not yet proven to result in mortality reduction or to benefit anyone, but that’s going to come in about five to 10 years,” he says.

In 20 years, he predicts, genetic screening that can inform patients of their cancer risk will be broadly available. While products claiming to do so are currently on the market, they’re not recommended for the general population, he notes.

For now, the greatest problem with MCEDs and genetic testing might not be their accuracy, but the fact that medical science simply isn’t ready for them.

Says Hawk: “They identify things that we don’t have solutions for.”