Originally published 2021-09-15.
In September 2021 England's National Health Service began recruiting for what it described as the world's largest trial of a blood test designed to detect many types of cancer before symptoms appear. The test, called Galleri and made by the US company GRAIL, looks for tiny fragments of tumour DNA in the bloodstream and claims to pick up signals from more than 50 cancer types. The full results, presented in 2026, were mixed: the trial missed its main goal, but showed fewer cancers being found at the most advanced stage.
How the Galleri test works
All cells shed small pieces of DNA into the blood when they die. Tumour cells do too, and their DNA carries distinctive chemical tags, known as methylation patterns, that differ from those of healthy tissue. Galleri sequences this circulating "cell-free" DNA and uses a machine-learning model to look for a cancer pattern.
If it finds one, it returns a "cancer signal detected" result and a prediction of where in the body the signal probably comes from, such as the bowel, pancreas or lung. A positive result is not a diagnosis. It has to be followed up with scans, scopes or biopsies to confirm whether cancer is really present.
The appeal is obvious. National screening programmes in England cover only a few cancers (breast, bowel, cervical and, in some groups, lung). Many of the deadliest cancers, including pancreatic, ovarian and oesophageal, have no routine screening at all and are often found late, when treatment options are poorer.
How the NHS-Galleri trial was designed
The trial was run by NHS England with GRAIL and run day to day with researchers at King's College London and Cancer Research UK. Volunteers were invited from the general population, with no cancer symptoms, and in the end more than 140,000 people aged 50 to 77 took part (142,250 enrolled, according to GRAIL).
Participants gave a blood sample once a year for three years and were randomly split into two groups. In one group the samples were tested with Galleri straight away and anyone with a cancer signal was referred for investigation. In the other, samples were stored, and those participants continued with usual care and standard screening. Comparing the two groups shows whether the test genuinely changes outcomes, not just whether it finds cancers.
The main question was whether testing would reduce the number of cancers diagnosed at a late stage (stage 3 or 4). Cancer deaths are being tracked over a longer follow-up. Professor Peter Sasieni of King's College London, one of the trial's leaders, stressed at the launch that the test had to be studied carefully to find out whether it could significantly reduce late-stage diagnoses.
Early hopes and a cautious pause
The 2021 launch was full of optimism. NHS chief executive Amanda Pritchard called the test potentially "the beginning of a revolution in cancer detection and treatment", and NHS England had signalled that, if early results were strong, it could expand use of the test in a large pilot before the trial ended.
That did not happen. In 2024 NHS England said the first year of data was not compelling enough to justify an early large-scale rollout, and that it would wait for the final results, expected in 2026, before considering any wider use. The UK National Screening Committee, which advises on all national screening programmes, would also need to review the evidence.
What the final results showed
GRAIL presented the full results at the American Society of Clinical Oncology meeting in May 2026. According to the company's summary:
- The primary goal was not met. There was no statistically significant reduction in combined stage 3 and 4 diagnoses for the 12 pre-specified deadly cancers.
- Fewer stage 4 cancers. Stage 4 diagnoses were about 14% lower in the testing group overall, with larger reductions in the second and third rounds. This was a secondary outcome and only nominally significant.
- More early cancers found. Stage 1 and 2 diagnoses rose by about 16%, and the test group had roughly four times as many cancers detected by screening as standard screening alone.
- Accuracy. About half of people with a positive result turned out to have cancer, and false positives occurred in roughly 0.45% of tests. The test correctly predicted the cancer's origin in about 92% of confirmed cases.
- Missed cancers. The test caught only around 31% of all cancers that arose during the screening period, so a negative result is not an all-clear.
These figures come from the manufacturer and had not yet appeared in a peer-reviewed journal at the time of presentation. Independent experts and the UK National Screening Committee will judge whether the benefits are large enough to justify national use.
Common misunderstandings about multi-cancer tests
"It diagnoses 50 cancers." It looks for signals linked to more than 50 types, but sensitivity varies widely, and it is much better at spotting advanced cancers than small early ones.
"A negative result means I'm cancer-free." No. Most cancers in the trial were not detected by the test, so existing screening and attention to symptoms remain essential.
"Finding more cancer always saves lives." Not necessarily. Screening can detect slow-growing cancers that would never have caused harm (overdiagnosis), and false positives lead to anxiety and invasive tests. That is why trials measure late-stage diagnoses and deaths, not just detection rates.
What this means for you
For now Galleri is not part of the NHS screening programme. It is sold privately in some countries, including the United States, where it is not FDA-approved but is available as a laboratory-developed test. Anyone considering a private multi-cancer test, or worried about their cancer risk, should talk to a doctor first.
The most reliable steps remain taking part in the national screening programmes you are invited to and getting persistent or unusual symptoms checked promptly. Multi-cancer blood tests may still find a place alongside them, and longer follow-up from the NHS-Galleri trial, including effects on deaths, will be key to that decision.
Sources: NHS England: An update on the ongoing NHS-Galleri trial; GRAIL: full results from the NHS-Galleri trial (May 2026); NHS England: trial launch announcement (September 2021); original reporting by Reuters.
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