New approaches to colorectal cancer screening

It could be time for a collective sigh of relief, or at least a fingers-crossed kind of hope, because the process of screening for colorectal cancer (CRC) might soon get easier. The latest evidence shows that a simple blood test offers promise as a CRC screening tool. And that's significant, since many people shy away from current screening methods — such as a colonoscopy or a stool test — because they can be unpleasant and time-consuming.

Skipping the screenings can have deadly consequences: CRC is one of the most common causes of cancer death in the United States.

Here's what the new blood test might mean for CRC screening, and where we stand on other methods.

Blood test in the offing?

The blood test is called Shield, made by Guardant Health. The test looks for DNA fragments shed by CRC tumors. Shield is already commercially available (though not widely) with a doctor's order, but it's not FDA-approved, and it's not covered by insurance. That could be changing.

An industry-funded study published March 14, 2024, in The New England Journal of Medicine evaluated Shield's accuracy on more than 7,800 adults who were also undergoing colonoscopies. All of the participants were at average risk for CRC.

Overall, Shield accurately identified 83% of CRC cases.

However, the test wasn't great at finding precancerous growths called polyps (Shield found only 13% of advanced polyps). And about 10% of the time, the test gave a falsely positive result in people who did not have CRC or polyps.

Still, if Shield receives FDA approval, which could happen this year, the test might be a game changer for people who can't or don't want to have other types of screenings. "It's not the best test from a performance point of view, but it's better than no test, because it might pick up colon cancer before it spreads. And if it can be done on a large scale because it's easier than other screening methods, that would make it the holy grail of cancer screening. But it's too early to tell if that will happen," says Dr. Lawrence S. Friedman, a gastroenterologist and the Anton R. Fried, M.D., Chair of the Department of Medicine at Harvard-affiliated Newton-Wellesley Hospital.

Stool tests

Stool tests don't require you to get your blood collected at a laboratory or doctor's office. Stool tests come in a kit that your doctor prescribes. You complete the collection in the privacy of your home and send the sample to your doctor's office or a lab. There, it's analyzed for signs of colorectal cancer, such as tumor DNA or tiny amounts of blood from tumors or polyps.

The U.S. Preventive Services Task Force recommends three types of stool tests:

• A guaiac fecal occult blood test (gFOBT) uses chemicals to find blood in the stool. This test must be done once a year.
• A fecal immunochemical test (FIT) uses antibodies to detect blood in stool. It must be done once a year.
• A multitarget stool DNA (mt-sDNA) test (Cologuard), also known as a FIT-DNA test, identifies DNA from cancer cells in the stool and has a FIT component to look for blood. This test must be done every three years.

Of the three, the FIT-DNA test is the most effective and currently more accurate than the blood test. It identifies 92% of CRC cases and 42% of advanced polyps. Its false-positive rate is about 13%.

The next generation of the Cologuard FIT-DNA test, which is awaiting FDA approval, is even better. It can find 94% of CRC cases and 43% of advanced polyps, according to a study published March 14, 2024, in The New England Journal of Medicine. Its false-positive rate is about 10%.

The gold standard screening

The most comprehensive CRC screening is a colonoscopy, which allows your doctor to peer inside your colon and rectum. This type of screening finds 95% of CRC cases. "It's not a perfect test, but it's more accurate than a blood or stool test. The doctor can find and remove potentially precancerous polyps on the spot, and prevent future cancer," Dr. Friedman says.

Unfortunately, getting a colonoscopy requires a lengthy, uncomfortable prep to clean out the gut (so your doctor can see the walls of your colon and rectum clearly). The procedure usually takes place in an outpatient surgery center. During the colonoscopy, while you're under sedation, the doctor places a flexible tube (which carries a tiny camera and surgical equipment) through the rectum and into the colon. After the screening, you must spend time in a recovery room and then get a ride home. The whole process is time-consuming, and it can be expensive, depending on your insurance.

Costs

The Affordable Care Act requires that Medicare and private insurance pay all costs of a CRC screening (a colonoscopy or one of the recommended stool tests) for preventive care.

However, if you opt for a stool test and it finds something suspicious, you'll need a follow-up colonoscopy, which is considered a diagnostic test. Diagnostic colonoscopies aren't covered completely by insurance and can cost hundreds to thousands of dollars, depending on your plan. Yet because they also enable the doctor to remove precancerous polyps, they also can be lifesaving.

What if you don't have insurance? Costs for one of the stool tests range from about $30 to $600. Costs for colonoscopies average between $1,200 and $4,800.

The Shield blood test is currently $895, and patients must pay the entire fee. If Shield receives FDA approval, the price would likely go down. But a suspicious finding from the blood test would require a colonoscopy follow-up, which would rack up more costs.

Unanswered questions

As exciting as a simple blood screening for CRC may be, we still have many unanswered questions about it.

For example, we don't yet know how often screening would be necessary. The manufacturer recommends that the test be taken every three years, like the FIT-DNA test, but there is not yet a formal recommendation for the blood test. For a colonoscopy, someone at average risk for colon cancer should get the screening every 10 years.

It's also unclear if a blood test would truly be cost-effective, given its false-positive rate, or if it would truly be convenient. "If you have to get a blood test every few years, but you only need a colonoscopy once every 10 years, the time commitment and logistical aspects might eventually even out," Dr. Friedman says.

We also don't know if the blood test would work well in younger people. In the 2024 study of Shield, participants were ages 45 to 84.

Where does that leave us?

At the very least, we can be confident that we already have good CRC screening tests, and hopeful that a simpler screening method might be on the way.

"Many people would probably prefer a blood test for CRC screening, but it's not quite ready for prime time. The current findings about Shield need to be confirmed by more studies," Dr. Friedman says. "Do I think a stool or blood test will ever be as good a screening test as a colonoscopy? I'm not sure, because there's no substitute for getting a good look inside the colon. And not all cancers shed genetic material that shows up in the blood. But who knows? Maybe we'll find a marker one day that's always present long before a cancer is discovered. Ultimately, the best test to screen for CRC is the one that you'll get done."

Image: © Richard_Pinder/Getty Images