The idea is compelling: could a single blood test look for signs of many different cancers before symptoms appear?
One of the world's largest randomized studies of that approach has now produced an important but mixed answer.
The NHS-Galleri trial included more than 142,000 adults aged 50 to 77 and tested whether adding the Galleri multi-cancer early-detection blood test to usual healthcare could shift cancers toward earlier diagnosis.
The trial did not meet its primary endpoint.
There was no statistically significant reduction in the combined incidence of stage III and stage IV cancers in the group offered Galleri testing compared with usual care.
However, some secondary findings were more encouraging, including a reduction in stage IV cancers.
The results therefore do not show that multi-cancer blood testing is useless — but neither do they establish Galleri as a proven replacement or addition to routine population cancer screening.
How does Galleri work?
Cancers can release fragments of DNA into the bloodstream.
Galleri analyses cell-free DNA from a blood sample, looking for patterns of DNA methylation associated with cancer.
Methylation refers to chemical modifications of DNA that can affect how genes behave.
The test uses these patterns not only to identify whether a cancer signal may be present but also to predict the likely cancer signal origin — the part of the body from which the signal may have arisen.
A positive result is not itself a cancer diagnosis.
It must be followed by appropriate diagnostic investigation.
Likewise, a negative Galleri result does not rule out cancer.
What did the NHS-Galleri trial test?
The NHS-Galleri study was a large randomized controlled trial involving approximately 142,250 people aged 50–77 who did not have a known suspicion of cancer when they entered the study.
Participants were randomly assigned either to usual NHS care or to usual care plus repeated Galleri testing.
The central question was not simply whether the blood test could detect cancer.
Researchers wanted to know whether using it in a population-screening setting could reduce the number of cancers diagnosed at advanced stages.
That distinction is important.
A screening test can detect biological signals without necessarily improving meaningful outcomes for the people being screened.
What happened to the main endpoint?
The primary endpoint assessed the incidence of stage III and IV cancers.
There were approximately:
- 706 stage III or IV cancers in the intervention group
- 688 in the control group.
The incidence rate ratio was approximately 1.03.
In other words, the study did not demonstrate a statistically significant reduction in its primary combined late-stage-cancer endpoint.
This is the most important result and should not be obscured by more favourable secondary analyses.
Was there any encouraging result?
Yes.
When researchers examined stage IV cancer separately, the Galleri group had fewer stage IV diagnoses.
The reported incidence rate ratio was approximately 0.86, corresponding to about a 14% relative reduction.
That is potentially important because stage IV cancer has spread beyond its original site and is generally more difficult to treat.
However, this was a secondary finding.
A secondary endpoint can generate important evidence, but it should not be treated as though the trial's prespecified primary endpoint had succeeded.
Most importantly, the current evidence does not establish that Galleri screening reduces cancer-specific mortality.
That question requires longer follow-up.
What does PATHFINDER 2 add?
A separate large prospective study called PATHFINDER 2 provides additional information about how Galleri performs in an intended-use population.
Among more than 32,000 performance-evaluable participants, the test had very high specificity.
Reported performance included approximately:
- 99.64% specificity
- 60.3% positive predictive value
- 39.3% overall episode sensitivity.
Sensitivity was higher — approximately 69.8% — for a prespecified group of 12 cancer types.
When a cancer signal was detected, the test predicted its likely origin correctly in approximately 91.3% of cases.
These figures illustrate both the promise and the limitation of the technology.
A highly specific test produces relatively few false-positive cancer signals.
But an overall sensitivity of about 39% also means that many cancers were not detected by the blood test.
What do sensitivity and specificity mean?
Sensitivity describes how often a test correctly identifies people who actually have the disease.
A test with imperfect sensitivity can miss cancers.
Specificity describes how often a test correctly identifies people who do not have the disease.
High specificity helps reduce false alarms and unnecessary diagnostic investigations.
For a population-screening test, both matter.
Even a technically impressive test can cause harm if it produces too many false positives, while a test that misses too many cancers may provide false reassurance.
Should people replace existing cancer screening with Galleri?
No.
Current evidence does not support replacing established screening programmes with Galleri.
People should continue guideline-recommended screening appropriate to their age and risk, such as established breast, cervical, colorectal or lung-cancer screening where applicable.
FDA's own proposed-use material for Galleri states that a result of No Cancer Signal Detected does not rule out cancer and that individuals should continue recommended single-cancer screening.
The multi-cancer test is being considered as an additional screening strategy, not as a substitute for proven screening programmes.
Why is the negative primary endpoint important?
Medical tests should ultimately improve outcomes, not merely produce interesting laboratory results.
The NHS-Galleri trial therefore asked a much harder question than whether the technology can find cancer-associated DNA.
It asked whether offering the test across a population could meaningfully change when cancers are diagnosed.
The failure to significantly reduce the combined stage III and IV endpoint means that the most ambitious hypothesis tested by the trial was not demonstrated.
At the same time, the reduction in stage IV cancers means the result is not simply a story of failure.
It is a mixed result that requires further follow-up.
What about FDA review?
On 23 September 2026, the FDA's Molecular and Clinical Genetics Panel of the Medical Devices Advisory Committee met to consider GRAIL's premarket approval application for Galleri.
The proposed indication concerns adults aged 50 years or older and the detection of cancer-specific methylation patterns in blood.
The advisory committee is being asked to consider the clinical significance of Galleri's sensitivity, specificity and performance across cancer types and stages, as well as the overall benefit-risk balance.
An FDA advisory committee provides expert recommendations to the agency.
Its recommendation is not the same as FDA approval.
As of publication, the official FDA meeting page lists the meeting materials and voting questions but does not provide an authoritative vote tally or panel recommendation. Galleri has not received FDA approval.
What is the most accurate conclusion?
Galleri represents an ambitious attempt to detect many cancers through one blood sample.
Large prospective studies show that it can identify cancer signals with high specificity and often predict where those signals originated.
But the large randomized NHS-Galleri trial did not significantly reduce its primary combined endpoint of stage III and IV cancers.
The reduction in stage IV cancers is encouraging and deserves further investigation, but it does not yet demonstrate that screening with Galleri saves lives.
The central question therefore remains open:
Can detecting multiple cancers through blood testing ultimately reduce deaths and improve outcomes enough to justify population-wide screening?
Longer follow-up and regulatory assessment will be crucial to answering it.
