The Food and Drug Administration (FDA) has approved Shield, the first colorectal cancer blood test designed to screen for colorectal cancer. Developed by Guardant Health, this test is now available for adults aged 45 and above who are at average risk for colorectal cancer. While the Shield test is not a replacement for colonoscopies, it provides a valuable alternative for patients and could help address the low testing rates for colorectal cancer, which is the second leading cause of cancer-related deaths.
Expanding Screening Options with the Colorectal Cancer Blood Test
Despite several available tools to screen for colorectal cancer, only about 60% of eligible U.S. adults get tested. Many find colonoscopy preparation burdensome and the procedure uncomfortable, while others dislike handling stool samples for stool-based tests. The introduction of the colorectal cancer blood test Shield aims to make screening more accessible by offering a simple blood test that can be done during a routine office visit.
“The beauty of having this new colorectal cancer blood test available is the ease with which people can be screened for cancer,” said Craig Eagle, MD, Chief Medical Officer at Guardant Health. “Wherever a blood test is available, you can now get screened for colon cancer.” By increasing screening rates, early detection of colorectal cancer can be improved, potentially saving lives. Over three-quarters of colorectal cancer-related deaths occur in those who are not up to date with screening.
How the Colorectal Cancer Blood Test Works
Patients will likely access the Shield test through their primary care physician during annual physicals. The colorectal cancer blood test requires no special preparation or dietary changes. It detects small DNA fragments released into the bloodstream by dying cells. Colorectal cancer patients often have higher concentrations of these DNA fragments. The Shield test analyzes these fragments to determine if they originate from cancer cells.
Comparison to Existing Colorectal Cancer Screening Tests
Cologuard, a non-invasive test approved by the FDA a decade ago, analyzes stool samples for signs of blood and cancer cell DNA fragments. In contrast, the Shield colorectal cancer blood test detects DNA fragments in blood, a more complex process. Larger cancer lesions shed more cells, making blood tests more likely to detect advanced cancers rather than early-stage disease. Clinical trials have shown that Shield was particularly effective at identifying late-stage cancers.
Effectiveness and Accuracy of the Colorectal Cancer Blood Test
Guardant conducted a randomized control trial with nearly 8,000 participants at average risk for colorectal cancer. The colorectal cancer blood test detected cancer in about 83% of the 65 participants with the disease, missing 17% of cases. Shield was particularly effective at identifying stage 2 and 3 cancers, detecting all such cases, but identified only 65% of stage 1 cancers and 13% of advanced adenomas.
David Lieberman, MD, a gastroenterologist at Oregon Health and Science University, led an expert panel discussing how Shield should integrate with existing colorectal cancer screening tools. He noted that while the blood test reduces the risk of developing and dying from colorectal cancer compared to no screening, it is less effective at cancer prevention due to its lower detection rate of advanced polyps and early-stage cancers.
Addressing False Positives with the Colorectal Cancer Blood Test
In clinical trials, about 10% of cancer-free participants received a false positive result from the colorectal cancer blood test. False positives were more common in older adults, likely due to DNA damage associated with aging. William Grady, MD, Medical Director of the Gastrointestinal Cancer Prevention Program at Fred Hutch Cancer Center, explained that ongoing research aims to improve the accuracy of blood-based cancer screening tests by detecting additional substances in the blood.
Despite the false positive rate, the Shield colorectal cancer blood test can streamline the screening process by encouraging those with the highest likelihood of abnormal colonoscopy findings to get the procedure. “Even if 10% still get a colonoscopy and they’re found to have nothing, you made the whole screening process a lot more efficient,” said Howard Hochster, MD, FACP, Associate Director for Clinical Research at the Rutgers Cancer Institute.
Not a Replacement for Colonoscopy
While Shield is an excellent screening tool, it is not a diagnostic test. Abnormal results from the colorectal cancer blood test require a follow-up colonoscopy to confirm the presence and location of cancer. Studies show that only about half of individuals who test positive for cancer on a stool test get a colonoscopy within a year. Encouraging follow-up colonoscopies is crucial for effective colorectal cancer screening.
Individuals at higher-than-average risk should prioritize colonoscopies over stool or blood tests to remove early-stage cancer growths and polyps that could become cancerous. Even those at average risk have a 1 in 20 chance of developing colon cancer in their lifetime, emphasizing the importance of effective screening. For the latest health news follow WS Coverage.
