A Blood Test for Cancer?
For many types of cancers there is no reliable screening method, and unfortunately once symptoms develop, it’s often too late. In recent years, a new form of cancer screening technology has been developed that aims to screen for dozens of types of cancer using a single blood test.
How Does it Work?
The technology centers around detecting cell-free DNA, which are fragments of DNA that are shed by dividing tumors into the bloodstream. The best-established test in this space is the Galleri® test, made by an American company called GRAIL.
The Pathfinder Study
The original study validating the Galleri® test was called the PATHFINDER study, which was released in the Lancet in 2023.1
This study showed 74% sensitivity for 12 deadly cancers, and a positive predictive value of 43%, meaning that 4 out of 10 times a test is positive – cancer is diagnosed.1 In the cancer world, where many cancer screening tests have a positive predictive value as low as 5%, this is actually relatively good. The test is better for detecting more actively dividing cancers that are shedding more tumor DNA into the bloodstream and is thus more likely to miss slow-growing, or indolent tumors.
PATHFINDER 2 is currently underway, looking at adding the Galleri® test to standard cancer screening in over 35,000 adults.2 According to a recent press release from the company this June, provisional results are showing “substantially higher positive predictive value” compared to the original PATHFINDER study.3
Where Can You Get the Test?
This technology is not yet readily available in Canada, but can be coordinated through centers in the States. If further promising data continues to come out, this form of technology may become a widespread tool for screening for cancer in years to come.
References
1. Schrag D, Beer TM, McDonnell III CH, Nadauld L, Dilaveri CA, Reid R, Marinac CR, Chung KC, Lopatin M, Fung ET, Klein EA. PATHFINDER: A Prospective Cohort Study of Blood-Based Multi-cancer Early Detection. Lancet (London, England). 2023 Oct 7;402(10409):1251.
2. PATHFINDER 2: a multi-cancer early detection study. ClinicalTrials.gov. Updated December 30, 2024. Accessed June 19, 2025. https://clinicaltrials.gov/study/NCT05155605
3. GRAIL announces positive top-line results from the Galleri PATHFINDER 2 registrational study. News release. Grail Inc. June 18, 2025. Accessed June 19, 2025. https://grail.com/press-releases/grail-announces-positive-top-line-results-from-the-galleri%e2%93%a1-pathfinder-2-registrational-study/