It’s good news in the fight against colorectal cancer: A new test can detect the disease with a simple blood draw.
But don’t cancel your colonoscopy just yet—that old standby is still more reliable and effective than newer screening methods. That’s because a colonoscopy doesn’t just detect cancer; it can prevent it.
“It’s exciting to have another tool to screen for colorectal cancer,” says UNC Health gastroenterologist Zachary Spiritos, MD. “The concern is that people will think it’s a substitute for a colonoscopy, and it’s not.”
Dr. Spiritos answers questions about the blood test, available under the brand name Shield, and other screening options.
How does the Shield test work compared with other colorectal cancer screening methods?
Those screened with the Shield test, approved by the Food and Drug Administration in July 2024, have their blood drawn in a doctor’s office or clinic. The blood is tested for biomarkers and DNA mutations that are present with cancer.
Another option, Cologuard, can be done at home—you collect a sample of your stool and mail it to a lab with a special kit. The sample is tested for blood abnormalities and DNA mutations.
During a colonoscopy, your doctor inserts a tube with a camera into your rectum and colon and looks for signs of cancer, such as polyps or irritated tissue. The procedure requires light anesthesia, which means you have to take time off from work or school, and the preparation involves following a liquid diet and taking a laxative so that your colon is empty, giving your doctor the best view.
Unlike a colonoscopy, neither Shield nor Cologuard requires any special preparation, dietary modifications or sedation.
“Shield reflects some of the current obstacles of screening,” Dr. Spiritos says. “There are people who are unwilling to undergo a colonoscopy and who are deterred by providing a stool sample,” as required with Cologuard.
In comparison, many people are used to providing a blood sample at their primary care provider’s office, so using Shield may seem like an easy way to check off colorectal cancer screening during an annual physical.
Is the Shield test as effective as other colorectal cancer screening methods?
Dr. Spiritos is concerned about Shield’s effectiveness at detecting colorectal cancer and advanced precancerous polyps called adenomas, compared to the other methods.
“With a colonoscopy, the rate of detection of colorectal cancer is over 95 percent, and the rate of detection of adenomas is 90 percent,” he says. “Cologuard has a 92 percent rate of detection for colorectal cancer, and a 45 percent rate of detection for adenomas.”
Shield has an 83 percent rate of detection of colorectal cancer, and a 13 percent rate of detection for adenomas.
Because of their low sensitivity to adenomas, Shield and Cologuard both miss important opportunities to prevent cancer altogether.
“A colonoscopy is both a diagnostic test and a therapeutic procedure because we can remove those polyps and adenomas when we see them, before they turn into cancer,” Dr. Spiritos says. “That prevents the development of cancer and the psychological burden of a cancer diagnosis. It saves you from surgery, chemotherapy and time away from work and your family.”
It’s also important to note that if your Cologuard or Shield test is positive, you will then need a colonoscopy, so the less invasive options may add an extra step. Dr. Spiritos says that’s relevant given the current false-positive rate of Shield.
“Shield has a false-positive rate of 10 percent,” Dr. Spiritos says. “That means 1 in 10 people will think they have cancer when they don’t. That causes anxiety, unnecessary stress, and you’ll have to get a colonoscopy anyway.”
Who is a good candidate for the Shield blood test?
While Dr. Spiritos wants people to be aware of the limitations of Shield, the blood test will make testing for colorectal cancer more available for patients unwilling or unable to get a colonoscopy or use Cologuard. Only about two-thirds of people eligible for a colonoscopy get one, he says.
“This could be used in remote areas where it’s challenging to get a colonoscopy,” Dr. Spiritos says. “It may also be beneficial for people with a developmental or cognitive impairment that makes it difficult to collect stool.”
Shield is similar to Cologuard in that it will be recommended if you are at average risk for colorectal cancer, meaning you are not experiencing any symptoms related to colorectal cancer and do not have a family history of the disease. If you have a first-degree relative who was diagnosed with colorectal cancer or who had adenomas, you would not be eligible for either Shield or Cologuard.
What are the current recommendations for colorectal cancer screening?
If you are at average risk for colorectal cancer, meaning that you aren’t experiencing any symptoms and don’t have a family history, you should begin regular screening for colorectal cancer at age 45. If you have a colonoscopy, your doctor will let you know when you will require another depending on the presence of polyps; generally, you need one every 10 years if you remain at average risk. If you have a Cologuard test, that should be repeated every three years. Dr. Spiritos says it remains to be seen how often a Shield test will be recommended.
While most people begin screening for colorectal cancer at the age of 45, if you have a family history of colorectal cancer, you may need to begin screening sooner; speak to your doctor about your personal risk.
The rate of colorectal cancer has been rising in adults younger than 50, which is why it’s important for people to be aware of early symptoms of colorectal cancer. Dr. Spiritos says to tell your doctor if you have changes in your bowel habits, abdominal discomfort or pain, unintended weight loss, blood in your stool or unexplained anemia. You may require earlier screening.
As you consider your screening options, Dr. Spiritos emphasizes that while a colonoscopy seems the most difficult, it provides the most comprehensive benefits.
“It’s a vulnerable test, because you have to be partially undressed in a room full of strangers, and you’ve had to fast and be on the toilet for a long period of time beforehand,” he says. “But it’s really safe, you don’t feel anything, and after it’s over, you can go out to eat whatever makes you happiest and rest well knowing your risk for colorectal cancer has been significantly reduced.”
If you’re concerned about your risk for cancer, speak to your doctor. If you need a doctor, find one near you.