FDA Approves Blood Test for Colon and Rectal Cancer
Fast Facts
- The Shield test by Guardant Health is a new blood test for colon and rectal cancers (colorectal cancer) which detects cancer DNA (cfDNA) circulating in the blood.
- Shield is very sensitive for detecting Stage II, III, IV colorectal cancers, but very limited for polyps and Stage I cancers. It needs to be repeated every 3 years.
- Shield isn’t for everyone. It’s not recommended as a screening test for anyone who has a family history of colorectal cancer, a family or personal history of certain genetic disorders, a personal history of adenomas or other positive screening test.
- Colonoscopy is still the gold standard for screening for colon and rectal cancer. The best screening test is the one that you’ll actually do and stay up to date with.
A few months ago, CheckIt4Andretti reported on an exciting new blood test for colon and rectal cancers (CRC). The test wasn’t FDA-approved at that time, but recently it jumped the last hurdle and now Shield by Guardant Health has been recognized as a valid test option. Many people don’t follow through with other CRC screening tests like colonoscopy, FIT or radiographic tests because they seem complicated or kind of gross. New blood screening tests are quick and easy. And now that they have FDA approval, they can be covered by Medicare and insurance. Let’s take a look at the advantages and drawbacks to ctDNA blood testing as a screening test for CRC.
cfDNA tests explained
All cells use DNA as an instruction manual for making the proteins that control metabolism, growth and cell division. Cancer cells use the same kind of DNA as healthy cells. But changes in DNA (mutations) allow cancer cells to grow at a much faster rate than normal, break free of neighboring cells and spread to parts of the body where they don’t belong. When the fast-growing cancer cells die, they break open and spill their DNA into the bloodstream. Doctors can check the blood for cancer DNA using special tests that look for mutations.
cfDNA testing is also called a liquid biopsy because it uses blood (a liquid) instead of the traditional solid tumor biopsy. It is much easier and less expensive to draw a vial of blood than to do a tumor biopsy. A phlebotomist can draw blood with tests in a lab, whereas a trained professional like a surgeon is needed for a solid tumor biopsy.
You might see terms like cfDNA (cell-free DNA) or ctDNA (circulating tumor DNA) along with liquid biopsy. The terms aren’t exactly the same, but they’re all ways of describing a way of looking for small fragments of DNA circulating in the blood. cfDNA can come from normal cells or cancer cells. ctDNA comes only from tumor cells. There are circulating DNA tests for many different kinds of cancers, prenatal tests and inherited diseases.
Is a liquid biopsy better than the other tests?
Yes and no.
Yes, because Guardant Health reports a 90% follow-through rate for their liquid biopsy. All other screening tests have about a 50% follow-through rate. More than 1 in 3 Americans are behind on recommended screening for CRC. If you’re more likely to follow through with a test, then it’s better than the one you won’t follow through with.
No, because liquid biopsy doesn’t pick up polyps or Stage I cancer very well. That’s because there’s not a lot of cancer DNA in the blood early on. And cfDNA testing isn’t for everyone. Among the people who cfDNA tests aren’t indicated include:
- People who’ve recently had a different abnormal screening test, like stool tests or radiology tests
- Those with a family history of colon cancer
- Individuals with risk factors like IBD, Lynch Syndrome or Familial Andenomatous Polyposis
- People who have a history of colon cancer, adenomas or related cancer
If you have a positive cfDNA test, you will need to follow-up with a colonoscopy. That’s because cfDNA tests like Shield only tell you IF you have cancer. It doesn’t tell you what stage or how aggressive the cancer is. Only a colonoscopy with a biopsy can give you the full picture.
Colonoscopy is still the gold standard for CRC screening. But if you’re one of the 50% who’s unlikely to get a colonoscopy or do a stool-based test, then a blood test is a great option.
Take the Colon Cancer Risk Assessment
How good is the cfDNA test?
The data for Shield is very good, but not as good as a colonoscopy. The later the stage, the more likely you’ll get a positive test. Shield’s sensitivity for each stage are as follows:
Sensitivity measures how likely a test is to detect an illness if that illness is present.
The Shield test is 90% specific. That means that 1 in 10 people who do not have CRC will test positive. A positive screening test is always a reason for a colonoscopy.
The Bottom Line
Colorectal cancer has become the second deadliest cancer for men and women. Screening can detect CRC early when it’s most curable. Right now, only about 50% of people follow through with getting their screening test done. New liquid biopsies like Guardant Health’s Shield test are quick, convenient and affordable. They have a higher follow-through rate, meaning more people are likely to get the test done. The test isn’t for everyone, so talk to your doctor. If you’re reluctant to get a stool based test or colonoscopy, a simple blood test may be the answer you’ve been looking for. Remember, a lot of people are counting on you, so get checked out. Check it for the people who love you. CheckIt4Andretti.
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