Only about two-thirds of the people who should have a colonoscopy actually get one. In the quest to improve colorectal cancer screening rates, companies are developing tests that are less invasive.
The first blood test to detect colon cancer and rectal cancer, called Shield, was approved in July 2024 by the U.S. Food and Drug Administration (FDA) for adults 45 and older who are at average risk of colorectal cancer.
But gastroenterologist Robin Mendelsohn, MD, and her colleagues at Memorial Sloan Kettering Cancer Center (MSK) don’t recommend the Shield test as a screening tool. Dr. Mendelsohn, who is Co-Director of MSK’s Center for Young Onset Colorectal and Gastrointestinal Cancers, is an expert in colorectal cancer screening.
“We used to say the best test is the one that gets done,” she says, “but we’re hesitant to even say that about this test because there are so many others that do a better job.”
Here, Dr. Mendelsohn answers common questions about Shield and other colorectal cancer screening tests.
How is Shield different from other screening tests for colorectal cancer?
Shield is a blood test, also known as a liquid biopsy. Liquid biopsies are based on the idea that because tumors shed their DNA into the bloodstream, cancer genes can be detected with a simple blood draw. This genetic material is called cell-free DNA (cfDNA) or circulating tumor DNA (ctDNA).
What have studies shown about the effectiveness of the Shield test?
A clinical trial published in the New England Journal of Medicine (NEJM), found that across all stages of cancers, Shield detected 83% of cases, which means it missed more than 15%. This is OK, but not great.
Unfortunately, Shield was much less effective at detecting stage 1 colorectal cancers — missing 35% of them. That’s important, because the goal of screening tests is to catch cancers early, when they are more treatable.
For precancerous polyps, the findings were worse, with Shield finding only 13% of them.
Also troubling, less than half of people with suspicious results from a Shield test get a follow-up colonoscopy within six months to check for cancer, according to a recent study in Gastroenterology.
Are there any other blood tests to detect colorectal cancer, besides Shield?
No liquid biopsy tests, other than Shield, are FDA-approved for colorectal cancer, but there are some that are being studied.
Another blood test being developed for colorectal cancer, called Freenome, appears to be slightly less effective than Shield.
There are also liquid biopsy tests that aim to detect a wide range of cancers, including colorectal cancer. The most commonly known cancer blood test being studied is called Galleri.
These tests are not considered to be reliable screening tools by gastroenterology and colorectal cancer screening experts.
What is required for a liquid biopsy to be considered a reliable screening test for colorectal cancer?
A good screening test needs to have high sensitivity — meaning it can detect cancer even at very low levels. It must also have high specificity — meaning that it detects only cancer and not other conditions.
For a screening test to be effective for colon and rectal cancers, it must be able to detect precancerous polyps as well as cancer. When these polyps are found early, we can remove them before they develop into cancer.
How does Shield compare to the Cologuard test?
Approved by the FDA a decade ago, Cologuard is more effective than Shield. The latest version of the Cologuard test detects 94% of colorectal cancers, according to a 2024 NEJM study.
Like Shield, Cologuard looks for cancer genes. But Cologuard looks for cancer DNA fragments in the stool rather than in the blood.
It makes sense that a test analyzing stool would be more effective than a blood test for colorectal cancers. This is because colorectal cancers form on the inner intestinal wall, allowing stool to pick up bits of tumor DNA as it passes through the colon.
Larger polyps are also more likely to shed their DNA to the stool. But Cologuard finds only 42% of polyps, which is a much lower rate than we would like to see, especially when compared to a colonoscopy.
For people who can’t or won’t have a colonoscopy, the Cologuard test and other stool tests are better than blood tests or no colorectal cancer screening test at all.
Why does MSK still recommend colonoscopies as the best way to screen for colorectal cancer?
Colonoscopy is the only test that’s been shown to prevent cancer, because it detects and removes polyps, which can be tested for cancer.
People avoid colonoscopies largely because they are worried about the prep. But if the results from a blood or stool test indicate that there may be something wrong, you will need to get a colonoscopy anyway.
At the same time, a DNA stool test or liquid biopsy may suggest that you don’t have cancer or polyps when you actually do. This gives the false impression that you don’t need to have a colonoscopy and gives the cancer a chance to grow and spread.
You can learn more here about other types of colorectal screening tests recommended by MSK.
What is the future of liquid biopsies that could screen for colorectal cancer and other cancers?
Obviously, we would love to have blood tests that could detect cancer early, before there are any signs or symptoms. But as of 2025, we are not there yet.
I think it will be especially challenging to develop blood tests that can detect polyps and early-stage colorectal cancers. That’s largely because of the anatomy of the gut and how these cancers form: Once the genetic material has made it to the bloodstream, the cancer is much more likely to be invasive and advanced.
