Can a Blood Test Detect Colon Cancer?
Blood tests offer a less invasive way to screen for colorectal cancer, but colonoscopy remains the gold standard
That dreaded reminder just arrived: time for your colonoscopy. You likely know screening is important, and skipping it could mean missing the chance to catch colorectal cancer (CRC) early, when it's most treatable. But the bowel preparation alone is enough to make you want to put it off, not to mention the time off work and the procedure itself.
Colorectal cancer is the second most common cause of cancer deaths among men and women in the United States. Death rates have been declining in older adults for decades, largely thanks to increased screening and early detection. But CRC rates among those younger than 55 have been increasing — a reminder that colorectal cancer can affect people of all ages.
"Colorectal cancer is one of the most curable cancers, yet research shows tens of millions of people are skipping out on potentially lifesaving screening," says Lance Uradomo, M.D., a board-certified interventional gastroenterologist at City of Hope in Orange County, California.
Other screening options like stool-based tests — including fecal immunochemical tests (FIT) and Cologuard — can be done at home, but they're not for everyone. Collecting and mailing your own stool samples isn't exactly pleasant. And there's the risk of forgetting to complete the test altogether.
But what if there was another option, such as a simple blood test?
There is. A new blood test for colon cancer screening, Shield, received FDA approval in July 2024, joining Epi proColon, which has been available since 2016. These tests offer a less invasive way to screen for colon cancer, but how well do they work? And if a blood test can detect colon cancer, does this mean you can skip a colonoscopy?
How They Work
Epi proColon and Shield look for pieces of DNA floating in your bloodstream, called cell-free DNA, that come from cancer cells, Uradomo says. "These might indicate if a person may have cancer or pre-cancerous growths in the colon."
Each test works a bit differently. The Epi proColon test looks for methylation (a chemical change in the DNA) in a gene called SEPTIN9, which often shows up in colorectal cancer.
The Epi proColon test detects these DNA changes with about 68% sensitivity and 79% specificity, says Jeremy Kortmansky, M.D., clinical director of the GI Medical Oncology Division at Yale Cancer Center. This means that out of 100 people with colon cancer in clinical studies, this test correctly identified 68 but missed the cancer in the other 32. And among people without cancer, about 1 in 5 had a "false positive" result — meaning the test suggests they might have cancer when they don't.
The newer Shield test looks at three aspects of DNA in the blood: changes in the DNA sequence, abnormal chemical tags on the DNA and patterns in how the DNA is fragmented.
In a recent clinical trial, Shield found cancer in about 8 out of 10 people who had stage I to stage III colorectal cancer (those that had not spread to other organs), Kortmansky says. But the test still incorrectly flagged 1 in 10 people as having cancer when they didn't, with a false positive rate of 10%. Shield was much better at identifying cancer than catching precancerous polyps, missing about 9 out of 10 polyps.
The testing process for both options involves a routine blood draw at your health care provider's office, followed by laboratory analysis. Medicare covers blood-based tests for CRC, but private health plans may have different coverage requirements, so it's worth verifying with your insurer.
One major advantage of a colonoscopy is that health care providers can remove precancerous polyps during the procedure.
Blood tests offer a less invasive way to screen for CRC, but colonoscopy remains the gold standard, says John Nathanson, M.D., gastroenterologist and assistant professor of medicine at Columbia University Irving Medical Center. Colonoscopy is a much more sensitive test, detecting approximately 95% of colon cancers in some studies, he explains. This means it's much more likely that a cancer or an advanced precancerous polyp will be caught, he adds.
One major advantage of a colonoscopy is that health care providers can remove precancerous polyps during the procedure, preventing them from developing into cancer, Uradomo says. He notes that blood tests aren't as accurate as stool-based tests, which can detect 91% to 92% of colon cancers but are also limited in detecting precancerous polyps.
Who Blood-Based Tests Are For
Blood-based colon cancer tests are intended for people at average risk, explains Uradomo. This means you might be a candidate if you don't have a personal or family history of colorectal cancer, no previous precancerous polyps and no genetic predisposition to the disease.
The Epi proColon test is recommended for average-risk individuals 50 and older, while Shield is approved for those 45 and older. This aligns with current guidelines that recommend starting colorectal cancer screening at age 45, as early detection significantly improves your chances of successful treatment.
These blood tests may be useful for older adults or those with medical conditions for which a colonoscopy poses increased risk, Kortmansky says. They may also be an option for people who simply don't want to have a colonoscopy.
While blood tests are FDA-approved, they're not a replacement for established screening methods.
That said, blood tests for colon cancer screening aren't widely endorsed by medical authorities. "These tests are currently not recommended by the panels that review methods of cancer screening," Nathanson points out. He emphasizes that while blood tests are FDA-approved, they're not a replacement for established screening methods like colonoscopy, stool tests or computed tomography (CT) colonography. The major gastroenterology societies haven't recommended blood tests as alternatives because the research is still limited, and they're not as cost-effective as other approved screening methods.
Despite this, medical experts acknowledge the potential benefits of these newer tests. Only about one-third of 45- to 75-year-old adults received colon cancer screening in 2023, says Uradomo, adding that if blood tests encourage more people to get screened, especially those who otherwise wouldn't, that's a step in the right direction.
But these tests have limits, Kortmansky cautions. Unlike a colonoscopy, where gastroenterologists can find and remove precancerous polyps, blood tests primarily screen for cancer. If a colon cancer screening blood test comes back positive, you'll likely still need a colonoscopy to confirm the findings and take appropriate action, he adds.
What’s Next?
Considering a blood-based colon cancer screening? Start by talking with your health care provider, who can help determine if it's a good fit based on your health history and risk factors. Plus, since these tests aren't available over the counter, you'll need your provider to order one for you anyway.
A negative result doesn't completely rule out colon cancer or precancerous polyps. That's why experts recommend repeating colon cancer screening blood tests every three years and following your provider's guidance on other screening options.
Since these tests aren't available over the counter, you'll need your provider to order one for you.
And a positive blood test isn't a diagnosis, but it does require prompt action. Nathanson suggests following up with your health care provider immediately after a positive result to determine your next steps, which typically include a referral to a gastroenterologist.
Colorectal cancer screening saves lives. No matter which screening method you choose, what's most important is taking that first step and getting screened regularly. If you've been putting off screening, today's the day to make that appointment and start the conversation with your health care provider. The sooner you get screened, the better your chances of catching potential issues early, when they're most treatable.
