For decades, colorectal cancer screening has relied on colonoscopies and stool-based tests. While these methods are effective, many people have avoided screening due to discomfort, cost, or limited access to specialists. As a result, millions of eligible adults remained unscreened despite the very large importance of early detection. Researchers have long searched for a simpler way to detect colorectal cancer, and recent advances in blood-based screening may offer a promising solution.
In July 2024, the FDA approved Shield, the first blood test authorized as a primary screening option for colorectal cancer in average-risk adults. The approval represents a significant step toward making cancer screening more accessible to the general public.
Detecting cancer through a blood sample
Cancer can be detected through a blood sample, because cancer cells do not remain isolated within a tumor. As the cells grow and die, they release small fragments of DNA into the bloodstream. Scientists refer to these small fragments as circulating tumor DNA, or ctDNA.
Shield works by analyzing a blood sample for specific genetic and epigenetic patterns that are associated with colorectal cancer. This test is very easy to complete, because no bowel preparation, sedation, or specialized procedures are necessary. Instead, this test can be completed during a routine blood draw.
The Data
Before receiving FDA approval, Shield underwent evaluation in the ECLIPSE study, one of the largest clinical trials conducted for a blood-based colorectal cancer screening test. The study enrolled more than 20,000 adults between the ages of 45 and 84 who were considered at average risk for colorectal cancer. Participants received both the Shield blood test and a colonoscopy, allowing researchers to directly compare the new technology with the current standard.
The results showed:
- Shield correctly identified 83% of colorectal cancer cases.
- The test also detected 13% of advanced precancerous lesions.
These findings suggest that blood-based screening can identify many cancers, though it remains less effective than colonoscopy at finding precancerous growths before they became cancerous.
Purpose of Shield
The significance of Shield extends beyond the technology itself. One of the greatest challenges in colorectal cancer prevention is a lack of participation, and many individuals delay or avoid screening because they are uncomfortable with existing screening options. Other people face logistical barriers such as scheduling difficulties, transportation issues, or limited access to healthcare facilities.
A blood test like Shield offers a potentially simpler alternative. For patients who would otherwise skip screening, the ability to add cancer screening to a routine medical visit could lead to earlier detection and improved outcomes.
Disadvantages of Shield
One limitation of Shield is its relatively low detection rate for advanced precancerous lesions, which was seen in the study. Colonoscopy still remains valuable because physicians can identify these lesions and remove them, preventing cancer from developing.
In addition, a positive Shield result does not confirm a cancer diagnosis. Patients with positive findings might still undergo a follow-up colonoscopy to determine whether cancer is present.
For these reasons, blood-based screening should be complementary to colonoscopies, instead of a replacement for these existing methods.
The Future
Although many questions remain regarding effectiveness, Shield demonstrates that blood-based cancer screening is no longer a future possibility, but that it is already becoming part of modern healthcare. As scientists continue to refine these technologies, the most important impact may not be the test itself, but the new possibilities it creates for earlier, simpler, and more accessible cancer detection.
Links for further reading:
What is Shield? National Institutes of Health
Carroll CARES 