You Can Now Screen for Colorectal Cancer With a Blood Test—But Should You? Does that mean you shouldn't get it?
There's a new blood test to screen for colorectal cancer, but a new study found it's not as effective as a colonoscopy or stool test
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- A blood test is now available to screen for colorectal cancer.
- However, a recent study concluded that the test shouldn't replace traditional screening methods, such as colonoscopy or stool tests.
- People who would otherwise forego colorectal cancer screening may want to consider the blood test, experts said.
For years, the tools available to screen for colorectal cancer were colonoscopies and stool tests. That changed in August, when a new FDA-approved blood test to detect colorectal cancer became available at doctor’s offices.
Now, a new study has spotlighted that blood test, called the Shield test, and a similar test that has yet to hit the market. The research found that the blood tests may not prevent as many colorectal cancer diagnoses and related deaths as traditional screening methods.
So does this mean you shouldn’t opt for a blood test if you’re eligible to be screened for colorectal cancer? Here’s what you need to know.
What Is Colorectal Cancer—and How Can You Screen For It?
Colorectal cancer develops in the tissues of the colon or rectum and is the third most common cancer in the United States (excluding skin cancers). It has been on the rise among younger adults but is still relatively rare, with people younger than 50 having a 0.4% risk of developing it. Routine screening is recommended by the US Preventive Services Task Force beginning at age 45.
Of the screening options, colonoscopies are perhaps the most well-known—and most dreaded. They involve taking a laxative beforehand to empty the bowels and then a procedure in which a doctor inserts a long, thin tube equipped with a camera into the rectum. If no cancer or advanced precancerous lesions called polyps are found, most people can wait 10 years to get another one.
The other screening options are non-invasive but require more frequent follow-ups—and all require a colonoscopy if polyps or cancer is detected.
One option is a CT colonography, also known as a “virtual colonoscopy,” which uses X-rays to examine the colon and rectum every five years.
People can also choose stool tests, which they can do at home. There are two types: a fecal immunochemical test (FIT), a yearly test that uses antibodies to detect blood in the stool, and a FIT-DNA test (Cologuard), which looks for blood and altered DNA and gets repeated every three years.
The Shield blood test, manufactured by Guardant Health, is the latest screening tool to hit the market. It’s meant for people over age 45 at average risk for colorectal cancer. Like the blood test developed by biotechnology manufacturer Freenome, which is seeking FDA approval, it works by detecting circulating DNA that may come from cancers.
A study conducted last year that included nearly 8,000 participants found the Shield test was 83% successful at detecting CRC in an average-risk screening population and 13% successful at detecting polyps.
Comparing Screening Methods
The new Stanford research found that colonoscopies and, to a lesser extent, stool tests are vastly more effective than the Shield blood test.
To reach this conclusion, researchers used a computerized modeling study to project the number of CRC cases and deaths expected if people used each screening strategy—blood tests, stool-based tests, and colonoscopy—over the long term, explained Uri Ladabaum, MD, lead study author and a professor of gastroenterology at Stanford Medicine.
The team’s projections showed that of 100,000 people who might receive a colonoscopy every decade, 1,543 would develop colorectal cancer, and 672 would die from the disease. Those figures rose for stool-based tests, with researchers finding 2,181 to 2,498 cases per 100,000 people and 904 to 1,025 deaths.
Of the three screening methods, however, rates were highest for people using the blood tests. Under that projection, cases of CRC would range from 4,310 to 4,365, and deaths would rise to 1,604 and 1,679, roughly two and a half times as many deaths as in the colonoscopy group. Blood tests, which can be pricey and aren’t always covered by insurance, were also less cost-effective than other methods, the researchers found.
But blood tests still reduced cases and deaths compared to no screening at all: Under that scenario, researchers found that 7,470 people would develop the cancer and 3,624 would die from it.
The research confirms what’s already well-established: Colonoscopy is the gold standard of CRC screening. But, as the authors noted, it also sheds light on how the newly developed blood tests stack up against it.
“The main conclusion is that the first generation of blood tests can provide substantial benefits at acceptable costs compared with no screening,” Ladabaum said. “But that the currently available stool tests and screening colonoscopy are much more effective and cost-effective.”
Why Blood Tests Can Be Ineffective
The reason why blood tests aren’t as effective as other screening tools is that polyps and early-stage cancer don’t shed as much DNA as later-stage cancers, said Daniel S. Reuland, MD, a professor of medicine at the University of North Carolina School of Medicine and director of UNC’s Carolina Cancer Screening Initiative Intervention and Implementation Research Program. “Unfortunately, this means these blood tests tend to pick up more later-stage cancers and are not quite as effective as colonoscopy or stool tests at detecting early-stage cancers or CRC precursors,” he told Health.
“This matters,” he continued, “because finding and removing the advanced precancerous lesions can prevent—and reduce incidence of—CRC, and finding early-stage CRC when it is easily curable reduces CRC mortality.”
What to Consider Before Getting Screened
Qin Rao, MD, a gastroenterologist at Manhattan Gastroenterology, recommends a colonoscopy for the average person but a stool test like Cologuard or CT colonography for people who lack access to healthcare or can’t otherwise get a colonoscopy.
If you switch from one screening method to another, Rao said you should screen at the appropriate intervals for that test. “For example, a person who is told they should get a colonoscopy every 10 years should not do a stool test before that 10 years if they choose to switch to stool testing in the future,” he said.
That said, it is possible to switch from stool tests to colonoscopies (and vice versa), though it’s less common, Rao noted. For example, a person might perform a Cologuard screening every three years, and if it turns positive—perhaps due to hemorrhoids—switch to colonoscopies afterward. “Likewise, a person who has had clean colonoscopies for several years might switch to stool tests in later age to avoid health complications associated with the procedure or due to limited healthcare access,” Rao said.
So, who should use the newly approved blood tests? At this point, Rao said, these blood tests are only indicated for screening in patients aged 45 and older with no family history of CRC. They shouldn’t be used with high-risk patients or those with alarming symptoms.
Blood tests are not an acceptable substitute for colonoscopies or stool tests, per Reuland, but he said they’d “be most beneficial for patients who are not getting any CRC screening because they have declined both stool testing with FIT or Cologuard and also prefer not to get a colonoscopy unless they really need it.”
Because they’re less invasive than colonoscopies and can be given more frequently, blood tests can also help monitor known cancers or assess risk, Maged Khalil, MD, a GI oncologist and associate director of research at Lehigh Valley Topper Cancer Institute, told Health.
For anyone interested in going the non-colonoscopy route, Reuland said to keep in mind that a positive test would prompt a recommendation for a colonoscopy anyway: “To be effective, all non-invasive tests require a follow-up colonoscopy if they are abnormal.”
This story originally appeared on: Health News - Author:Elizabeth Yuko, PhD
