A blood test to screen for colon cancer may be on the horizon

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The first blood test to screen for colorectal cancer could be approved this year — raising the potential to drastically increase the number of people diagnosed with the second-highest cause of cancer death in the United States.

The test would not replace the gold-standard colonoscopy, experts said, but could boost the dismal rate of colorectal screenings with a less invasive and perhaps more palatable way to detect colon cancer.

Colonoscopies and “stool-based tests have that kind of ‘ick’ factor related to them,” said Dr. William Grady, a gastroenterologist at Fred Hutchinson Cancer Center in Seattle. “We need to find tests that people are going to use.”

Guardant Health's Blood Collection Kit.
Guardant Health’s Blood Collection Kit.Guardant Health

Grady led the new research published Wednesday in the New England Journal of Medicine that found that a blood test called Shield from California-based Guardant Health was, overall, 83% effective in finding colorectal cancers.

That’s significant, said outside experts.

“We’ve been anticipating and waiting excitedly for a blood-based test for colon cancer,” said Dr. Aasma Shaukat, a gastroenterologist at NYU Langone Health. It’s been “a long time coming.”

Barriers to colon cancer screening

Colonoscopy is, by far, the most accurate and earliest way to detect cancers of the colon and rectum, as well as spots that may become cancer in the future. During the procedure, doctors go into the rectum with cameras in search of tumors and lesions that have the potential to become tumors if left alone.

Colon cancer screening is generally recommended starting at age 45, but less than 60% of people who are eligible are up to date.

“In an ideal world, everyone goes to get a first colonoscopy at 45. That’s not gonna happen,” said Dr. Tiago Biachi, a gastrointestinal oncologist at Moffitt Cancer Center in Tampa, Florida. Neither Biachi nor Shaukat were involved with the Guardant research.

Cue the “ick” factor, as Grady described it, as well as a number of other logistical barriers.

The day before a colonoscopy, patients must take laxatives to clear out their colon for cameras to get the best view. That is, they are stuck in a bathroom for several hours.

Patients usually need to take a day off of work for the actual procedure, and, because of the anesthesia that’s involved, they need someone else to drive them home afterward.

“The blood-based test doesn’t have any of those issues,” Grady said.

Guardant’s blood test works by detecting DNA released by cancerous tumors. It’s most effective in finding later stage cancers.

A positive test isn’t a diagnosis on its own and would need a colonoscopy as a follow-up. Only a colonoscopy can tell doctors where tumors are located in the colon, or guide treatment.

The location of colorectal tumors can determine whether patients need surgery, or chemotherapy and radiation before surgery, Biachi said. “This blood test is not a home run for cancer diagnosis.”

The Shield test was much less effective in detecting the earliest signs of colorectal cancer, when it’s most treatable. The test only found 13% percent of earlier stage polyps. That makes sense, considering that the test works by detecting tumor DNA. Precancerous polyps don’t release such DNA.

It could be a sticking point for doctors.

“Do I really want to tell a patient that, if this test is positive, I’ll be able to for sure detect a more advanced cancer, but I’m not so sure how well it does for precursor polyps or early cancer?” said Shaukat. “Those are the lesions that we’re really looking for where we can actually make a difference.”

The Food and Drug Administration is expected to consider the Shield blood test for approval sometime this year, according to Guardant Health.

If approved, the blood test would be given every three years, starting at age 45. People with symptoms of colon cancer, such as rectal bleeding, or a family history of the disease, would still need a colonoscopy.

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