You Can Now Get a Blood Test to Screen for Colorectal Cancer …C0NTINUE READING HERE >>>
Key Takeaways
The FDA approved the first blood test for screening colorectal cancer.In a clinical trial, the test identified 83% of the colorectal cancer cases and delivered false positive results to 10% of cancer-free participants.The blood test isn’t a substitute for a colonoscopy, but it could improve screening rates among people who are resistant to colonoscopy and stool-based tests.
The Food and Drug Administration (FDA) has approved the first blood test to screen for colorectal cancer. While the test isn’t a replacement for colonoscopies, it expands screening options for patients and could help improve poor testing rates for the second leading cause of cancer-related death.
The test, called Shield, was developed by Guardant Health and is now available to adults aged 45 and above who are at an average risk for colorectal cancer.
There are several available tools to screen for colorectal cancer, but only about 60% of eligible U.S. adults get tested. Some people find it burdensome to prepare for a colonoscopy and uncomfortable to go through the procedure, while others dislike handling poop for a stool-based test.
“The beauty of having this new test available is the ease with which people can be screened for cancer,” Craig Eagle, MD, chief medical officer at Guardant Health, told Verywell. “Wherever a blood test [is available] in the country, you can now get screened for colon cancer.”
Improving screening rates could help catch cancer early and save lives, he said. More than three-quarters of colorectal cancer-related deaths occur in people who are not up to date with screening.
However, a blood test isn’t a substitute for a colonoscopy. During a colonoscopy, a gastroenterologist can not only more accurately find early signs of colorectal cancer, but they can also remove cancerous and pre-cancerous growths, said Howard Hochster, MD, FACP, associate director for clinical research and director of gastrointestinal oncology at the Rutgers Cancer Institute, who is unaffiliated with Guardant.
“Colonoscopy is still the gold standard test, but a lot of people will not go for colonoscopy,” Hochster told Verywell. “The best screening test is the one that gets done.”
The test fits the criteria set by the Centers for Medicare and Medicaid Services for insurance coverage. Guardant said it expects Medicare coverage to be approved in the next few months and for commercial insurers to follow suit.
How the Test Works
Most people will likely access the Shield test through their primary care physician during a routine office visit, like an annual physical. However, it can be accessed just about anywhere that offers blood tests requested by a physician, Eagle said.
The patient doesn’t need to change their diet or prepare in any special way before the blood draw.
When cells die, they release small bits of their DNA into the bloodstream called cell-free DNA (cfDNA). People with colorectal cancer tend to have a higher concentration of those circulating DNA fragments. There are also differences in the way DNA is expressed in colorectal cancer cells compared to normal cells.
The Shield test looks for DNA fragments in the blood and analyzes whether that genetic information may have come from a cancer cell.
Cologuard, a non-invasive test that gained FDA approval a decade ago, works in a similar way. It analyzes stool samples for signs of blood and DNA fragments that may have come from cancer cells.
Stool passes right through the colon and rectum. Identifying fragments of DNA that can be traced back to the bowel from a tube of blood presented a particular challenge.
“That’s like finding a needle in a haystack,” Eagle said. “It takes a lot of time to get the technology to detect very small cancer molecules, DNA molecules, in the blood.”
When cancer lesions are big, they shed more cells than those that are less developed. That means a blood test is more likely to pick up signs of advanced cancers than early-stage disease.
How Well The Test Works
Guardant tested Shield in a randomized control trial of nearly 8,000 people. Those participants all had an average risk for colorectal cancer—they didn’t have blood in their stool or abdominal pain when they enrolled.
About 83% of the 65 people who had colorectal cancer got a positive result on the Shield test. The test missed 17% of those cases.
The blood test was particularly good at identifying late-stage cancers—every participant with a stage 2 or 3 colorectal cancer received a positive result. It picked up 65% of the stage 1 cancers.
Colorectal cancer often develops when noncancerous tumors, called adenomas, grow and become cancerous. In the clinical trial, the Shield test detected about 13% of advanced adenomas.
David Lieberman, MD, a gastroenterologist at Oregon Health and Science University and past president of the American Gastroenterological Association, led an AGA expert panel last fall to discuss how the Guardant test should fit with the existing colorectal cancer screening tools.
Modeling studies showed that taking the blood test “clearly” reduced the risk of getting and dying from colorectal cancer compared to no screening at all, he said.
However, Lieberman noted, “This test is not going to be really effective for cancer prevention because it’s going to fail to detect most patients that have advanced polyps, and it’s going to be less effective than some of the other tests are detecting earliest stages of cancer.”
“If patients substitute a blood test for one of the currently used and approved tests, that will result in more cases of colon cancer and more deaths from colon cancer,” he added.
Chances of False Positives
In the clinical trial, about 10% of the participants who were cancer-free received a false positive result from the blood test.
False positives were more common in older adults than in young adults. That finding is “not unexpected,” said William Grady, MD, medical director of the Gastrointestinal Cancer Prevention Program at Fred Hutch Cancer Center and senior author of the study.
DNA tends to become damaged as people age, so it can be more difficult to distinguish signs of cancer from normal aging in older adults.
“There is a lot of work going on in the field of blood-based cancer screening tests to improve on their current accuracy,” Grady told Verywell in an email. That includes finding ways to detect substances in the blood besides DNA, which can give a better picture of cancer in the colon.
In the Shield clinical trial, 4% of participants had colorectal cancer. With a 10% false positive rate, that means that more people had a false positive than a true result, Hochster said.
That’s not necessarily a bad thing, he said. Colonoscopies can be expensive and unappealing for many people. A blood test could filter out the people with the highest chances of having abnormal colonoscopy findings and encourage them to get the procedure.
“It would be a lot better for America if a lot of people that wouldn’t go for colonoscopy would get the blood test done,” he said. “Even if 10% still get a colonoscopy and they’re found to have nothing, you made the whole screening process a lot more efficient.”
Not a Perfect Substitute for Colonoscopy
While the Shield test can be an excellent screening tool, it’s not a diagnostic, Lieberman said.
When someone receives abnormal results from their Shield test, they need to get a colonoscopy to confirm the results and identify where there is cancer present in the bowel.
Studies show that only about half of people who test positive for cancer on a stool test go on to get a colonoscopy within a year. Encouraging people to get a follow-up colonoscopy “seems to be the Achilles heel of colon cancer screening,” Lieberman said.
Importantly, people who are at higher than average risk should get a colonoscopy rather than a stool or blood test, Lieberman said. That can help remove early-stage cancer growths and polyps that could turn into cancer.
Even the average risk individual has a 1 in 20 chance of developing colon cancer in their lifetime. “It’s not really a low risk,” Lieberman said.
What This Means For You
The U.S. Preventive Services Task Force and other major medical groups recommend people ages 45 and older who are at average risk get regularly screened for colorectal cancer. The regularity varies from every year for a stool-based FIT test to every 10 years for a colonoscopy. Some factors like medical conditions and age may influence which test is best for you. If you’re eligible for a colorectal cancer screening, especially if you are at a higher risk for the disease, talk to a health provider about the best screening test for you.
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