New blood test for colorectal cancer approved by FDA

A new blood test that screens for colorectal cancer has just been approved by the Food and Drug Administration (FDA).

The blood sample needed for the new test, called Shield, can be obtained at a doctor's appointment or through a commercial lab. The hope is that the test could boost the number of people who get screened for colorectal cancer, the second-leading cause of cancer-related deaths in the U.S. 

Shield is not considered an alternative to colonoscopies for diagnosing the disease. Rather, it's a less-invasive way to flag if someone likely has cancer so they can then be triaged for further testing. The new test has been approved for adults ages 45 and up with an average risk of colorectal cancer.

The five-year survival rate for colorectal cancer is very high — over 90% — when it's caught in its early stages, but that survival declines in advanced stages. Modeling studies suggest that, if gaps in colorectal cancer screenings were closed, around 70% of related deaths could be avoided. 

Related: Gut bacteria linked to colorectal cancer in young people

Recent data suggest that upwards of 1 in 3 eligible adults are not up-to-date on their recommended colorectal cancer screenings. 

"The persistent gap in colorectal cancer screening rates shows that the existing screening options do not appeal to millions of people," Dr. Daniel Chung, a gastroenterologist at Massachusetts General Hospital and a professor of medicine at Harvard Medical School, said in a statement from Shield's maker, Guardant Health. The Shield test offers a "compelling new solution to close this gap," he argued.

Shield works by screening the blood for bits of DNA shed by cancer cells and polyps, which are tissue growths in the colon that can be precancerous. In the results of a trial published in the The New England Journal of Medicine, the test was 83% sensitive for colorectal cancer, meaning it correctly identified the cancer most of the time. It missed 17% of colorectal cancer cases. 

In a separate test, 10% of people with no colorectal cancer got a false positive — so the test isn't perfect, but it is comparable to screening tests that involve taking a stool sample.

The clinical trial also found that Shield isn't great at detecting advanced precancerous polyps; it spotted them in only 13% of cases. That's why the test wouldn't be an appropriate replacement for more-invasive tests, such as colonoscopies, which involve using a long, thin scope to examine the entire colon and are very good at finding polyps. 

Thus, a person who tested positive for cancer on the Shield test would need a follow-up colonoscopy to confirm the diagnosis and locate the tumors.

Current guidelines recommend that people start getting screened for colorectal cancer at age 45, if they're at average risk of the disease. There are several options for screening: The stool-based tests should be used annually or every one to three years, depending on the specific test used, while colonoscopies are recommended every 10 years. A test that's used less frequently in the U.S., called flexible sigmoidoscopy, is recommended every five years.

Shield would be given every three years, NBC News reported. 

Now that it's approved, Shield is the second blood-based colorectal cancer test to be approved by the FDA. However, the first, Epi proColon, isn't covered by Medicare due to concerns over its accuracy and benefits. It's anticipated that both Medicare and private insurance will cover Shield, whose list price has yet to be announced. 

The new test is a "welcome development," Dr. Arvind Dasari, an associate professor in the Department of Gastrointestinal and Medical Oncology at the University of Texas MD Anderson Cancer Center, told NBC News. 

However, "we'll have to wait and see what the impact will be in terms of improving screening and reducing the incidence of mortality," he said.

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