A new blood test designed to detect signals from 50 types of cancer is generating cautious optimism among researchers. The Galleri test, developed by U.S. company Grail, is currently being trialed by the NHS and could eventually supplement routine cancer screening.
Early results from the Pathfinder 2 trial, involving 23,161 adults over 50 in the U.S. and Canada with no prior cancer diagnosis, suggest the test can identify cancer in some patients earlier than conventional methods. Of the 216 participants who tested positive, 133 were later confirmed to have cancer, producing a “positive predictive value” (PPV) of 62%.
The test also performed well at ruling out disease, correctly identifying 99.6% of participants without cancer. This high specificity could help reduce unnecessary follow-up procedures and anxiety. However, experts caution that even with strong specificity, widespread testing could generate tens of thousands of false positives in a population the size of the UK.
Sensitivity—the test’s ability to detect cancers that are present—was lower. In the trial, only 40.4% of cancers appearing within a year were detected, meaning roughly three in five cases went undetected. Low sensitivity raises concerns about false reassurance and delayed diagnoses for patients with negative results.
Statisticians note that PPV, specificity, and sensitivity are estimates, subject to variability, and real-world performance may be lower than in controlled trials. Additionally, there is currently no evidence that using the Galleri test widely reduces cancer mortality.
The Galleri test costs about $949 (£723) in the U.S., which may limit its immediate accessibility. Researchers say it could become a useful tool alongside traditional screening if negative results are interpreted carefully.
Experts emphasize that, while the early data are promising, the test is not a standalone solution. Its current limitations in sensitivity and cost mean it should complement, rather than replace, established cancer screening programs.
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