When colon cancer is found in its early stages, the five-year survival rate is 90%. Unfortunately, more than a third of eligible adults aren't up to date with their screening.
Could a new blood-based screening method make a difference? In summer 2024, the Food and Drug Administration (FDA) approved a blood test for colorectal cancer screening called Shield.
A simple blood draw certainly sounds far easier to complete than a colonoscopy, the traditional method of screening for colorectal cancer. But — yes, there's a but — this convenience comes with downsides.
"While we hope that the addition of blood tests will help increase the number of people screened for colon cancer, it's important for people to understand these tests are limited," says Dr. Monisha Singh, a gastrointestinal medical oncologist at Houston Methodist. "They are not likely to find precancerous colon polyps, and these tests typically only pick up cancer after it has already developed."
Colonoscopy, on the other hand, is highly effective at both finding and preventing colon cancer. That said, this new blood test can still play a role in colorectal cancer screening. Here's everything you need to know if you're considering one.
Available blood tests for colon cancer screening
Shield is a simple blood draw that requires no fasting, diet changes or preparation. It looks for specific changes to DNA floating freely in the blood, known as cell-free DNA. Certain changes in cell-free DNA can indicate a potential tumor in the colon. It's recommended that the Shield blood test be repeated every three years.
It's actually the second FDA-approved blood test for colorectal cancer screening. The first was Epi proColon, which was approved in 2016 but taken off the market in 2023, after the company that makes it went through restructuring.
Galleri, which is only available by prescription, is another blood test your doctor may suggest. It analyzes cell-free DNA to identify a molecular signal associated with more than 50 types of cancer, including colorectal cancer. A big caveat: Galleri isn't approved by the FDA.
"The hope is these blood tests are a less intimidating, more accessible way to be screened for colorectal cancer," says Dr. Singh. "But these tests aren't foolproof. They should be used along with other screening methods."
Limitations of blood tests for colon cancer screening
They may sound like a more enticing option, but blood tests for colon cancer come with some drawbacks.
For starters, they're only meant for people who are at average risk for colorectal cancer. If you have a family history of colon cancer or a pre-existing condition that increases your risk, you should not rely on one of these blood tests for screening. Talk to your doctor if you're unsure about your risk level.
Additionally, these tests aren't as sensitive as some other screening methods. A study showed that Shield detected colorectal cancers in 83% of trial participants in whom cancer was found during a colonoscopy, but its sensitivity for detecting precancerous growths was much lower, at only 13%. The test found all later-stage cancers, but only 65% of stage 0 and stage I cases — when cancer is preventable or much easier to treat.
"A limitation of this test, as well as other blood-based tests for cancer, is that it depends on the shedding of cancer cell DNA," says Dr. Singh. "You want to find cancer as early as possible — via precancerous polyps, in the case of colorectal cancer. At this very early stage, these blood tests aren't likely to effectively detect cancer."
Blood tests such as Shield also can't diagnose cancer. They can only help uncover whether a tumor may exist. This difference may seem subtle, but it means that further testing, typically a colonoscopy, is still needed after a positive blood test result. This is because of their tendency to return "false positive" results — suggesting cancer is present even though more sensitive follow-up testing goes on to confirm it's not. Shield has a false positive rate of 10%. So, one out of 10 people who do not have colon cancer or a precancerous polyp will incorrectly receive a "positive" result.
Lastly, health insurance coverage may differ for each test. Shield is covered by Medicare but may not be covered by all private insurers. Galleri, because it is not FDA-approved, is not covered by Medicare and is unlikely to be covered by private insurers. Check with your insurance provider before getting one of these blood tests, since they often come with high out-of-pocket costs.
Traditional screening methods for colon cancer
What's the best way to approach colorectal cancer screening then? It's recommended that people with average risk should begin colon cancer screening at age 45.
There are several types of traditional colon cancer screening methods:
Colonoscopy is considered the gold standard for colon cancer screening and prevention. During this procedure, your doctor will check the large intestine for cancer and polyps using a thin, flexible tube with a light at the end while you are under sedation.
The biggest complaint people usually have about colonoscopies is the preparation needed beforehand. You may need to drink large amounts of a laxative solution the night before, which can result in spending a lot of time in the bathroom. This is necessary to empty the colon and bowel, so the doctor can more easily examine them.
One of the biggest advantages to colonoscopy is that your doctor can remove any suspicious-looking areas, such as polyps, as they find them, and send them off for testing. (Related: 6 Questions About Colonoscopy, Answered)
Sigmoidoscopy is a similar, but less-invasive alternative to colonoscopy. This procedure only looks at the lower portion of the large intestine and rectum, instead of the entire large intestine. Like a colonoscopy, you will still need to do some bowel preparation beforehand to clean out the large intestine.
Stool-based tests look at your stool for possible signs of colorectal cancer or precancerous polyps. These tests are typically done at home and sent in for analysis, so many people find them easier to do. There are a couple different types of stool tests to screen for colon cancer:
- Cologuard checks for DNA changes and blood in your stool, which could be indicators of colorectal cancer or polyps.
- A fecal occult blood test (FOBT), such as a fecal immunochemical test (FIT), checks only for hidden blood in the stool.
If your stool-based test comes back positive, you will need to go in for a colonoscopy to look for polyps or other causes that account for the blood in your stool.
How often to screen varies by the test, your results and your risk for colorectal cancer. If your colonoscopy is negative, you likely can wait 10 years before the next one. Tests that use stool samples, however, must be repeated every one to three years.
The role of blood tests for colon cancer screening
While blood tests can be a valuable tool in colon cancer screening, they are not always foolproof and should be used along with other screening methods.
"Colonoscopy really is the gold standard when it comes to colon cancer screening," says Dr. Singh. "But there's a pretty significant chunk of the population not being screened because they don't want to go through the prep and the invasive procedure for colonoscopy. I think blood tests are a good option for patients who decline colonoscopy or perhaps are not candidates for medical reasons."
(Related: Why Even Young Adults Should Care About Colorectal Cancer Screening)
That said, most people are best served by a colonoscopy, not a blood test. A 2024 study found that if too many people switch to blood tests, colorectal cancer death rates will rise. Dr. Singh adds that they can be used instead to complement traditional screening methods.
Talk to your healthcare provider about what colon cancer screening test is best for you based on your history, risks and wishes.