What to know about the blood test to detect colon cancer recommended by an FDA panel

A blood test to screen for colorectal cancer is one step closer to receiving approval from the U.S. Food and Drug Administration.

An FDA advisory panel -- the Medical Devices Advisory Committee -- recommended FDA approval of the Shield blood test on Thursday night, finding it to be safe and effective and that the benefits of use outweighed potential risks.

Guardant Health, the company that makes Shield, is seeking FDA premarket approval for the test to screen for colorectal cancer among people aged 45 and older at average risk.

Shield is not the first blood-based screening tool available for colorectal cancer, but it presents another option to screen for a type of cancer that has been affecting a large number of Americans at younger ages than before.

Shield is already commercially available, but Guardant Health says FDA approval would broaden availability and coverage as another screening option.

Here's what you need to know about the tests and how it could help detect colorectal cancer. Guardant Health said it believes the FDA will decide on whether Shield receives premarket approval later in 2024. FDA approval isn't guaranteed. The agency usually agrees with its advisors, but not always.

What is colorectal cancer?

Colorectal cancer starts in the colon or the rectum. It may be referred to as colon cancer or rectal cancer depending on where the cancer originates.

Colon cancer and rectal cancer are commonly grouped together because they have many similar features, according to the American Cancer Society.

Most colorectal cancers are caused by polyps, which are growths on the inner lining of the colon or rectum. While most polyps are benign, others will turn into cancer over many years.

Signs and symptoms include a change in bowel habits, rectal bleeding, blood in stool, abdominal pain, weakness, fatigue and sudden weight loss, the ACS said.

While cancer deaths have declined overall, a report earlier this year from the ACS found a troubling increase in colon cancer in younger adults. Colon cancer is currently the No. 1 cause of death among men younger than age 50, and the No. 2 cause of death among women of the same age group.

How is colon cancer typically screened for?

Colonoscopies remain the gold standard when screening for colorectal cancer. During this procedure, a doctor inserts a colonoscope -- a long, thin and flexible tube with a camera on the end -- through the anus to examine the rectum and colon. Your doctor may biopsy any suspicious-looking tissues or polyps.

Additionally, stool tests can be performed to look for blood not visible to the naked eye, which is possibly a sign of cancer, according to the ACS.

The United States Preventative Services Task Force currently recommends adults aged 45 and older with average risk to be screened for colorectal cancer. This includes a stool test every one to three years or a colonoscopy every 10 years for people that do not have symptoms.

How does Shield work?

Shield detects colorectal cancer by detecting DNA shed by tumors in blood samples. Results takes about two weeks after the samples are received by the laboratory.

In clinical trial data, published in The New England Journal of Medicine, Shield was found to have a 83.1% sensitivity rate, meaning 91% of patients with colorectal cancer detected by a colonoscopy tested positive on the Shield blood test. It also had an 89.9% specificity rate, meaning 89.9% of patients without cancer tested negative on the blood test.

During the FDA advisory panel meeting, eight of the FDA independent advisers voted yes and one voted no on safety. On efficacy, six voted yes and three voted no. On benefits outweighing risks, seven voted yes and two voted no.

The company says the blood test will help eliminate barriers linked to current screening methods such as colonoscopies or stool-based tests, which can be time-consuming or difficult to complete. It says the blood test can be performed with any special preparation and as part of a standard doctor's visit.

"The advisory committee's strong support for the approval of Shield reinforces the crucial role that a blood test option can have in improving [colorectal cancer] screening rates for those at average risk," AmirAli Talasaz, co-chief executive officer of Guardant Health, stated in a press release. "Despite the importance of detecting [colorectal cancer] early, there are notable barriers that can deter average-risk Americans from completing existing screening methods."

"Providing people with this blood test alongside other noninvasive stool tests can increase the rate of colorectal screening and potentially reduce preventable [colorectal cancer] deaths," the statement continued.

Are there any drawbacks?

Despite the high overall sensitivity rate, the clinical trial data indicated Shield test may miss one in 10 people who have precancerous lesions and one in 1,000 people with cancerous lesions.

These "false negatives" may result in tests finding no evidence of cancer, but patients actually have pre-cancerous or cancerous lesions.

Data showed Shield also missed about 87% of advanced precancerous lesions, with a sensitivity rate of just 13.2%. Shield did not detect small lesions of less than 10 millimeters.

While the majority of the panel voted in favor of the test, some FDA advisers voiced concerns about appropriate patient counseling, patient misunderstanding of a "negative test" and packaging and/or labeling language.

ABC News' Youri Benadjaoud contributed to this report.