Can a Blood Test Spot Breast Cancer Before It Returns? What to Know

Justine Morris was 28 when she discovered a lump in her breast , and 30 when doctors pronounced her cancer-free. But after Morris's treatment had ended, every minor symptom caused panic: Was it a cold? Was she under the weather? Or had her cancer returned? "Because I have the BRCA2 gene, I have a higher chance of the cancer coming back," she says. "The type I had was fast-moving and aggressive." For patients like Morris, the uncertainty is a dreadful waiting game with physical exams, symptom monitoring, and yearly mammograms to detect any rogue cancer cells left behind after treatment. But she also does something more cutting-edge: Every three months, she has her blood drawn and sent to a lab for monitoring.

Morris is an early adopter of circulating tumor DNA (ctDNA) blood tests, offered through tests like Signatera as well as those from Personalis , as part of her breast cancer screening tool kit. Nearly 30 percent of women diagnosed with early-stage breast cancer will end up developing metastatic breast cancer, meaning it has spread to other parts of the body. Once cancer spreads, it is considered stage IV, and while treatable, it is incurable. Body scans can't catch stray cancer cells early, and aren't often used as a monitoring tool for some types of breast cancer. "In general, we don't scan people proactively," says Elisa Port, MD, a breast surgeon and author of The Breast Advice: All You Need to Know About Breast Health, Screening, and Treatment . "Because unfortunately, there is no such thing as early-stage IV disease."

But what if there was an easy way to detect a recurrence earlier? Doctors, researchers, and patients hope that ctDNA tests could hold the key. These tests, known as liquid biopsies, analyze a patient's blood to find residual cancer cells. They work by targeting someone's specific tumor DNA. "We take the patient's tumor, sequence it, and then look for mutations in the blood," explains Ben Ho Park, MD, PhD, a Breast Cancer Research Foundation (BCRF) scientist.

"Tumors shed pieces of their DNA into our blood," says Dorraya El-Ashry, PhD, chief scientific officer at BCRF. For a long time, ctDNA tests were not considered sensitive enough to detect that infinitesimal amount of cancer DNA and residual disease. It's harder than finding a needle in a haystack; it's like finding just the eye of it. But thanks to evolving biotech breakthroughs, the current generation of tests is equipped for the search. "It's like more shots on goal," Park says. "These tests can detect one cancer DNA molecule out of a million. The older-gen tests were a hundredfold less sensitive—one in 10,000."

In a study published in the Journal of Clinical Oncology this past March, Park and his team were able to determine the accuracy of an ultrasensitive ctDNA test from Personalis. Over the course of five years, doctors monitored 228 cancer survivors using ctDNA tests. Patients with triple-negative breast cancer who still had detectable ctDNA after surgery and other therapies were 128 times more likely to have their cancer return. "I had a young woman, a mother of two children, who had a little smidgen of one of her tumors [showing up in a blood test]," Park says. Her scans hadn't shown anything metastatic, but based on the information from the ctDNA, Park put her on an anticancer drug, and her scans have been clear now for 18 months and counting.

The next-gen tests are still in clinical trials, but a few are available for patients like Morris to use ahead of full FDA approval, with Medicare covering the cost for some cases. Signatera, in particular, is already premarket-approved for specific use cases like muscle-invasive bladder cancer. Still, some doctors are hesitant to introduce ctDNA tests quite yet to monitor for recurrences in people who have had breast cancer. The technology is "very promising," Port says, "but it is not yet proven to be ready for prime time. So what if you do a ctDNA test and it shows circulating tumor cells? Are you going to start someone on aggressive treatment based on those results alone?" In order for a ctDNA test to become standard, researchers will need to do more clinical testing to prove "beyond a shadow of a doubt" that it can actually extend a patient's lifespan, Park says. And while researchers haven't yet cleared that bar, Park and El-Ashry are both hopeful that in certain cases, more breast cancer survivors could be using the tests in the future.

Until there is a clear blueprint for introducing blood tests into cancer care, doctors will still follow standard screening and prevention protocols as well. And Port stresses the importance of sticking to the treatment plan recommended after surgery, since not doing so can up the risk of recurrence and potential metastatic disease.

Could tests like these become commonplace as preventive tools to detect recurrences of many kinds of cancer? We're not quite there yet, but it's possible. A trial conducted by the health care company GRAIL in partnership with the U.K. National Health Service did not prove that its multi-cancer early detection ctDNA test, Galleri, was able to find cancers before they are classified as stage III or IV (which was the end goal), but it did show that three years of screening cut the number of cancers found at stage IV in each sequential year. Park feels hopeful that we might see these tools used in clinical settings within 10 years or so as tests continue to improve.

In the meantime, the so-called liquid biopsies offer a glimmer of hope for cancer patients at a moment when they might otherwise feel totally in the dark. "I cannot tell you the mental weight of being a cancer patient and noticing everything about your body—the aches, the pains," Morris says. "Do I need to go get a scan? Now I have a built-in system in place where I know that I'm okay. I know if I do feel an ache or a pain, I'm getting a Signatera test in X amount of time, and I'm okay to wait until then. And that has been really helpful."

A version of this story appears in the October 2026 issue of ELLE.