The Food and Drug Administration (FDA) has approved Shield, a blood test developed by Guardant Health, as a primary screening tool for colorectal cancer in average-risk adults. This marks the first FDA-approved blood test for colorectal cancer screening and a potential shift in how the disease is detected and managed.
How Shield Works
Shield detects colorectal cancer by identifying specific DNA changes in cell-free DNA circulating in the blood. These changes are indicative of tumors or precancerous growths in the colon. The test aims to offer a less invasive alternative to traditional screening methods like colonoscopy and stool-based tests.
Clinical Trial Results
The FDA approval was based on data from the ECLIPSE study, which involved approximately 8,000 participants. The study found that Shield detected colorectal cancers with 83% sensitivity. However, the test's sensitivity for detecting advanced precancerous polyps was considerably lower, at around 13%. The test also demonstrated a specificity of 90%, meaning that 90% of individuals without cancer or advanced precancerous polyps received a negative result.
Daniel Chung, M.D., a gastroenterologist at Massachusetts General Hospital and lead investigator of the ECLIPSE study, noted that the test's sensitivity for precancerous polyps was lower than colonoscopy. However, he suggested that the convenience of a blood test could increase overall screening rates, potentially leading to a significant impact on public health.
Experts Weigh In
Asad Umar, D.V.M., Ph.D., of NCI’s Division of Cancer Prevention, who was not involved in the study, emphasized that while the Shield test is a major step toward closing the screening gap, important questions remain about its performance compared to other available options and its ultimate impact on preventing colon cancer deaths. He noted that factors such as testing frequency and cost will need to be considered.
John M. Carethers, M.D., of UC San Diego, highlighted that the blood-based nature of the test could increase screening rates across diverse populations. However, he also pointed out that a positive Shield test necessitates a follow-up colonoscopy, a step that is often not completed, potentially diminishing the test's overall effectiveness.
The Current Screening Landscape
The U.S. Preventive Services Task Force recommends regular colorectal cancer screening for adults aged 45 to 75 at average risk. While colonoscopy is considered the gold standard, its invasive nature and demanding preparation can deter many individuals. Stool-based tests like the fecal immunochemical test (FIT) and Cologuard offer more convenient options but are generally less accurate.
Despite high screening rates compared to other cancers, approximately 30% of eligible adults are not up to date with colorectal cancer screening. Colorectal cancer remains the second leading cause of cancer deaths in the United States.
Implications and Future Directions
The Shield test represents a significant advancement in colorectal cancer screening, offering a non-invasive option that could increase screening rates and potentially reduce cancer incidence and mortality. However, further research is needed to determine the optimal use of the test within the existing screening paradigm and to ensure that individuals with positive results receive timely follow-up care.
Guardant Health recommends Shield be administered every three years. Like stool tests, the blood test requires a follow-up colonoscopy if there’s an abnormal result, which could lead to more out-of-pocket costs.
Overall, experts agree that the Shield test has the potential to save lives, but its success will depend on addressing key questions about cost, follow-up care, and long-term effectiveness.
