Grail's multi-cancer early detection (MCED) test, Galleri, is poised to become the first blood test of its kind to gain Food and Drug Administration (FDA) approval, following a recent narrow positive recommendation from an agency advisory committee. The test relies on a single blood draw to analyze DNA fragments for common cancer signals and predict their primary source, while a positive result requires confirmatory tests such as imaging or a biopsy.
As the medical sector awaits the imminent regulatory decision and its potential implications for insurance coverage and the Medicare funding legislation anticipated in 2028, oncologists' opinions vary regarding the test's efficacy, accuracy, and the limitations associated with its use in daily clinical practice, with the listed price of the test standing at $949.
Accuracy Levels and False Positives
Dr. Elizabeth O'Donnell, director of the Early Cancer Detection Clinic at Dana-Farber Cancer Institute and assistant professor of medicine at Harvard Medical School, points out that the test's positive predictive value ranged between 50% and 62% in the NHS-Galleri and PATHFINDER 2 studies. Although the false-positive rate is very low (less than 1% of participants), the test's sensitivity window still ranges between 30% and 40%, indicating that some cases may evade early detection and require continuous scientific improvements.
For his part, Dr. Charles Atwood, head of the lung cancer screening program at the Pittsburgh VA Healthcare System, emphasizes that the improvement in positive predictive value in the PATHFINDER 2 study reflects the evolution of the test's accuracy, stressing the need for physicians and patients to understand current limitations and address tumors before they reach stage four.
The Difference Between Single-Cancer Screenings and Multi-Cancer Tests
Dr. Parijatam Thomas, medical director of the Cancer Prevention Center at MD Anderson Cancer Center, warns against viewing the test as a replacement for established standard screening methods, noting that the large-scale NHS-Galleri study did not meet its primary endpoint of reducing stage 3 and 4 cancer rates to a statistically significant degree.
Thomas explains that the test's efficacy varies significantly among different cancer types; it is very poor at detecting breast and prostate cancers, whereas traditional screenings such as mammograms and colonoscopies boast very high sensitivity, necessitating the continued performance of standard screenings.
Clinical Strengths and Risks
Dr. Kartik Giridhar, an oncologist at Mayo Clinic, identifies three positive aspects supporting the test, including a low false-alarm rate (based on a specificity rate of 99.6%) and high accuracy in predicting the cancer location by over 90%, which reduced subsequent invasive tests to just 0.6%. The test performs very well in detecting aggressive tumors that shed large amounts of DNA into the blood, such as liver, pancreatic, and colorectal cancers, while its efficacy remains low in slow-growing cancers.
Conversely, Dr. Arthur Burnett, a urologist and oncologist at Johns Hopkins University, warns of the risks of overdiagnosis and the potential financial and psychological ramifications associated with false-positive results, as well as challenges related to incorporating test results into health records and their potential impact on individuals' insurance coverage.