A new blood test offers some people another option for colorectal cancer screening, but medical professionals say it should not be viewed as a replacement for a colonoscopy.
Nikki Sommer, a nurse practitioner with Key West Surgical Group Inc. and Med Spa, joined Good Morning Keys on Keys Talk 96.9/102.5FM yesterday morning for Medical Matters.
Shield is a cell-free DNA blood test that looks for genetic changes and abnormal DNA patterns associated with colorectal cancer.
Sommer said, “All of us have tiny fragments of DNA circulating in our bloodstream. Cancer cells can release DNA fragments as well, so the Shield test looks for certain genetic changes and abnormal DNA patterns associated with colorectal cancer.”
The test provides a less-invasive screening option because it only requires a blood draw, but there are important limitations.
Colorectal cancer screening generally begins at age 45 for people at average risk. Those with a family history of colorectal cancer, particularly when a close relative was diagnosed before age 50, may need to begin screening earlier.
One of the primary purposes of screening is to find precancerous polyps before they become cancer.
Sommer explained that adenomas are precancerous polyps that can develop into cancer if they are not removed, while hyperplastic polyps generally do not become cancerous.
That is where the Shield test has a significant limitation.
According to Sommer, the major study supporting FDA approval found that Shield detected about 83% of colorectal cancers. However, its ability to detect advanced precancerous lesions was much lower, at about 13%.
“The big limitation is precancerous polyps,” Sommer said. “Shield was much less effective at detecting advanced precancerous lesions.”
That means a person could receive a negative Shield result while still having a potentially dangerous precancerous polyp.
“With a colonoscopy, we go in with a camera and we see if you actually have the polyps,” Sommer said. “When you do all these other things, you’re praying it detects something, but it doesn’t tell you necessarily whether you have a polyp or not.”
She also cautioned that the overall 83% cancer detection figure does not necessarily mean the test will detect cancers at every stage equally well. Another study cited during the discussion found that Shield detected about 65% of stage-one colorectal cancers.
The test can also produce false-positive results.
Sommer said that, similar to stool-based screening tests such as FIT and Cologuard, a positive Shield result does not automatically mean someone has cancer. It does mean additional testing is needed.
“If you have a positive Shield, just when you have a positive FIT or a positive Cologuard, guess what you’ve got to do? A colonoscopy.”
People should not assume that choosing the blood test means they will never need a colonoscopy.
“Don’t automatically assume the blood test is better simply because it’s easier,” Sommer said.
The Shield test is generally aimed at people at average risk who are 45 or older and have no symptoms and no family history of colorectal cancer or polyps, according to Sommer.
For people with a family history of colorectal cancer or polyps, a colonoscopy is generally the appropriate place to start.
The convenience of Shield is one of its biggest advantages. Unlike a colonoscopy, there is no bowel preparation, and the test can be ordered as a blood test through a healthcare provider.
Sommer said, “It’s great if people don’t want to have a colonoscopy, so it gives at least primary care provider some insight onto what’s going on, but again, if it turns out positive, you’re going to end up needing that colonoscopy.”
Cost can also be a consideration. Sommer said the test can cost approximately $1,495 for someone without insurance, although Medicare and many commercial insurance plans cover the test for eligible patients.
There is no single screening method that is perfect for everyone.
For someone who has a normal colonoscopy at age 45, with no polyps and no family history, she said the next colonoscopy may not be necessary for another 10 years. People who have polyps may need to be screened again in three to five years.
She said those using FIT, Cologuard or Shield generally need to repeat those tests every three years.
Sommer urged adults 45 and older who are at average risk to discuss their screening options with their healthcare provider.
“Don’t let looking for the perfect or easiest screening tool become the reason you don’t get screened at all,” Sommer said.
She also advised anyone experiencing gastrointestinal symptoms to speak with a healthcare provider rather than relying solely on a screening test.
The bottom line is that Shield can provide another tool for colorectal cancer screening, but it does not eliminate the need for colonoscopy in appropriate patients—particularly those at higher risk or anyone who receives a positive blood-test result.
For more information on how Key West Surgical Group can help, click here: https://www.keywestsurgicalgroup.com/