Colon Cancer Blood Test Shows Promise for Early Detection

Early detection of colon cancer can prevent a majority of deaths from this disease, possibly as much as 73 percent of them. But just 50 to 75 percent of middle-aged and older adults who should be screened regularly are being tested.

One reason, doctors say, is that the screening methods put many people off.

There are two options for people of average risk: a colonoscopy every 10 years or a fecal test every one to three years, depending on the type of test.

Or, as Dr. Folasade P. May, a gastroenterologist at UCLA Health puts it, “either you take this horrible laxative and then a doctor puts an instrument up your behind, or you have to manipulate your own poop.”

But something much simpler is on the horizon: a blood test. Gastroenterologists say such tests could become part of the routine blood work that doctors order when, for example, a person comes in for an annual physical exam.

“I think this is going to start taking off,” said Dr. John M. Carethers, a gastroenterologist and the vice chancellor for health sciences at the University of California, San Diego.

About 53,000 Americans are expected to die from colorectal cancer this year. It is the second-most common cause of cancer-related deaths in the United States, and while the death rate in older adults has fallen, it has increased in people under age 55.

Current guidelines recommend screenings starting at age 45. The problem is convincing more people to be screened.

Enter the blood test. It takes advantage of the discovery that colon cancers and large polyps — clumps of cells on the lining of the colon that occasionally turn into cancers — shed fragments of DNA into the blood.

A study published on Wednesday in The New England Journal of Medicine found that a blood test searching for such DNA called Shield and made by the company Guardant Health detected 87 percent of cancers that were at an early and curable stage. The false positive rate was 10 percent.

“This will be big news,” said Dr. May, who consults for Exact Sciences, the maker of the Cologuard fecal test.

But there is a caveat to the blood test: While it detects cancers, it misses most large polyps, finding just 13 percent of them. In contrast, the fecal test detects 43 percent and a colonoscopy finds 94 percent, Dr. Carethers said.

While polyps are usually harmless, a few can turn into cancers, so doctors want to find all of them and remove them to prevent cancers from forming.

Dr. Barbara Jung, the chair of the department of medicine at the University of Washington and president of the American Gastroenterological Association, said that patients should be fully informed before they opt for a blood test. In particular, they need to understand that while this test helps detect cancer early, it does not prevent it because it is not good at finding precancerous polyps.

“We have to have that discussion,” with patients, she said. But, she added, “a lot of this will fall on primary care physicians who are already very pressed for time to get through a litany of screens and counseling.”

Doctors will also need to explain to patients that if the blood test result is abnormal, they will have to schedule a colonoscopy to look for polyps or early-stage cancers and remove them if they are present.

It is also not clear how often people should do the blood test. Guardant suggested every three years but that recommendation is not well established, Dr. Jung said.

Dr. Jung added that she would love to know if the blood test performs well in people too young to qualify for existing screening tests. But that will require additional studies. She worries about rising colon cancer rates in younger people. It would be “very attractive,” she said, if people could get a blood test for colon cancer when they are in their 30s.

“That would be my biggest excitement,” Dr. Jung said.

The big unknown, though, is cost. Guardant has applied to the Food and Drug Administration for approval to market the test. The company sells it now as a “lab-based test,” which does not require F.D.A. approval but is also not covered by health insurance. For those who want to pay out of pocket, the price is $895. But the company will work with Medicare and Medicaid and private insurers to “finalize pricing” if it is approved, said Matt Burns, a Guardant spokesman.

Dr. William Grady, the medical director of the gastrointestinal cancer prevention program at the Fred Hutchinson Cancer Center who is a corresponding author in the trial sponsored by Guardant, said the company might settle on a price that makes it comparable to the price of the other screening methods. The Cologuard fecal test costs $581 to $681. Colonoscopies, usually needed half as often, typically cost $1,250 to $4,800, although some hospitals charge more. The average cost of a colonoscopy in the United States is $2,750. The tests are typically covered by insurance.

Dr. May cautioned that patients need to know that these three screening tests are not equivalent. With the blood test, she said, “we are moving from prevention to early detection.”

But, she said, the blood test is pretty easy. When a doctor is ordering routine blood work, all that needs to be done is to add a colorectal colon cancer test.

“That’s phenomenally exciting,” she said.

And its ease of use could outweigh the test’s limitations, said Dr. Carethers, who wrote an editorial in the journal accompanying the study. After all, he said, the goal of screening is to reduce deaths from colon cancer for the population as a whole.

If a blood test means many more people will be screened, the result — fewer deaths from colon cancer — will be beneficial.

“The best screening test is the one that gets completed by the patient,” he wrote.