Blood tests for colorectal cancer: What to know

The American Cancer Society recently released updated colorectal cancer screening guidelines for people at average risk for the disease. This includes adding blood tests as a new screening option.

Adults at average risk for colorectal cancer should begin screening at age 45. Blood tests are not meant to replace a colonoscopy, which is still considered the gold standard test for colorectal cancer. Blood tests provide an alternative for people who can’t or don’t want to get a colonoscopy or use other colorectal cancer screening options.

We spoke with Ernest Hawk, M.D., vice president and head of Cancer Prevention and Population Sciences, to learn more.

How do blood tests work?

Blood-based tests look for DNA changes in the blood that may indicate cancer. Blood tests for colorectal cancer screening must be prescribed by your doctor. You will get your blood drawn at your doctor’s office or another healthcare facility, and the blood will be sent to a lab for testing. A report will be sent to your doctor, who will go over the results with you.

“If the results are positive indicating possible colorectal cancer, you will need a diagnostic colonoscopy to examine your colon and find the underlying issue,” says Hawk. “If the results are negative, you should continue colorectal cancer screening as often as your doctor recommends.”    

Who should use blood tests — and who should not?

You should consider blood tests if you are at average risk for colorectal cancer and are not willing or able to use other colorectal cancer screening tests.

“For example, some people have trouble with the bowel prep required for a colonoscopy, or they’re not able to take a day off work to have the procedure done,” says Hawk.

You should not take a blood test in place of a colonoscopy if you are at increased or high risk for colorectal cancer. You’re considered high-risk if you have:

• A family history of colorectal cancer or colon polyps
• A personal history of colorectal cancer or colon polyps
• Inflammatory bowel disease like Crohn’s disease or ulcerative colitis
• Hereditary cancer syndromes, such as Lynch syndrome or familial adenomatous polyposis

If you’re at increased or high risk for colorectal cancer, you will need to be screened by colonoscopy. You will also need to start screening at an earlier age and be tested more often. Your doctor will recommend a personalized screening plan.

How often should blood tests be repeated to screen for colorectal cancer?

The American Cancer Society and UT MD Anderson recommend repeating blood tests every three years.

“It’s important to note that every three years is what’s recommended now, but that could change,” says Hawk. “We don’t yet have long-term outcome data to support that every three years is the best schedule for blood-based colorectal cancer screening because the blood tests haven’t been available for repeated testing over a long period of time. As we gather more data, that frequency could change.”

How accurate are blood tests for detecting colorectal cancer?

Accuracy is measured in terms of sensitivity. This is a measure of the test’s ability to correctly identify people with colorectal cancer.

Clinical study results for one FDA-approved blood test on the market show that it has 83% sensitivity.

“That’s quite good, but it’s not as accurate as colonoscopy or many stool-based tests, which have sensitivity of roughly 90% or higher,” says Hawk.

What are the limitations to blood tests for colorectal cancer?

Blood tests are not as good as colonoscopies or most stool-based tests for detecting stage 1 colorectal cancer or advanced adenomas (precancerous polyps).

“To find something in the blood means that the tumor has progressed to a certain stage for it to signal the DNA changes that blood tests can detect,” says Hawk. “Blood tests are very good at finding advanced-stage disease. But the whole point of screening is to find colorectal cancer early, when it’s easier to treat.”

In addition, if polyps are found during a colonoscopy, your doctor can remove them right then before they turn into cancer.

How much do blood tests cost?

Many insurance policies, including Medicare, will cover most or all of the cost of these blood tests.

Contact your health insurance provider to help determine coverage.

What happens if my blood test comes back positive?    

If your blood test comes back positive, you will need a diagnostic colonoscopy. This will allow your doctor to examine your entire colon and check for any issues, including precancerous polyps or cancer.

“It’s important to note that if you get a positive test result and your doctor finds that you have a colorectal polyp or cancer, you are no longer considered average risk,” says Hawk. “This means you’ll need to get a colonoscopy for any future colorectal cancer screenings at a frequency determined by your doctor.” 

What else should we know about blood tests for colorectal cancer screening?

Regular colorectal cancer screening can help find cancer early, when the chances for successful treatment are greatest.

Young adults are increasingly being diagnosed with colorectal cancer. If you’re younger than 45, check your family history for colorectal cancer and watch out for symptoms, such as:

• Blood in your stool
• Bloating
• Abdominal pain or cramping in your lower stomach
• Changes in normal bowel habits (size, shape and frequency)
• Fatigue

Your doctor may decide that you should start screening earlier.

“Colonoscopy is the gold standard test for colorectal cancer screening,” says Hawk. “But if you find that to be a significant barrier because of bowel prep, needing to take off work, having someone drive you to and from the appointment or for some other reason, then blood tests are a wonderful option.”

Some people are also not comfortable with the idea of collecting their own stool for a stool-based test. Blood-based tests provide another option for people who might not get screened otherwise.

“A blood test is certainly better than no screening at all,” says Hawk, adding that he expects blood-based screening will continue to improve over time. “I anticipate more blood-based tests will be in the market in the next few years. So, we can continue to increase the number of people getting screened for colorectal cancer.”

Request an appointment at UT MD Anderson online or call 1-877-632-6789.