I think it is fair to say that we all get slightly excited when we hear about new tests that promise an earlier diagnosis of cancer. There was, therefore, understandable interest when recent reports in the news suggested a possible new breakthrough in cancer screening.
The Galleri Test, run by an organisation called Grail, is a test that offers the tantalising potential to detect up to 50 different cancers. Dr Patrick Ruane has already written about another similar so called multi-cancer early detection test (MCED), the Trucheck test, about which there has also been significant excitement. Our conclusion here, after reviewing the evidence and consulting colleagues, was that despite its promise, it remains too unreliable to have a serious role in screening.
What is the Galleri test?
It is a blood test which the company says promises to detect tiny amounts of DNA in the blood circulating outside of blood cells. The test looks within this DNA for abnormalities associated with cancers, and when identified, runs further tests on that DNA to inform where in the body that DNA has come from. It hopes to detect up to 50 different cancers.
Why has the Galleri test attracted attention?
This test is already commercially available in the USA, priced at about $1000. There has been a significant trial running in the NHS of the Grail cancer test. Over 100,000 participants are taking part. It is not available in the UK outside of the trial currently. The trial will be completed in 2026, and if successful, a pilot of up to 1 million participants is being discussed. A considerably smaller trial in the USA, the Pathfinder 2 trial, is also ongoing.
How accurate is the Galleri test?
After reading into the details of the test to date and the initial outcomes of the NHS-Grail trial, I sadly think the conclusions will be disappointing. The test detects just 27.5% of early stage cancers (stage 1-2) which are most meaningful in terms of curative treatment. ‘Strikingly low results’ was how one professor of genetics labelled this data.
False Negatives
The number increases to 52.8% when just 12 cancers are singled out (mainly upper GI cancers such as liver, pancreas, biliary and oesophageal); however, for these cancers, the prognosis is poor regardless of stage. The conclusion is therefore that a false negative result is a real possibility.
False Positives
What of the false positives? A positive result that turns out to be false will inevitably only turn out to be false after extensive and often quite invasive testing and all the psychological impact of that testing. A false positive result for the Galleri is in the region of 0.5%. Across a population that number would be significant – leading to many thousands undergoing unnecessary and likely invasive further testing.
What do the current trial results mean?
Ultimately a successful screening test results in fewer deaths through meaningful earlier detection and fewer late stage cancer diagnoses. It is too early to know the answer to that yet for Grail. Screening is complicated as exemplified by ovarian screening which while resulting in cancer being detected at an earlier stage ultimately did not translate into reduction in cancer specific mortality. There so far is nothing to suggest that the Grail test is any better than the others in the field of multicancer early detection and the roll out of the trial into a pilot for a further million has been postponed.
Will Galleri be available in the UK?
It seems at this stage that the test is unlikely to be wheeled out nationwide as a cancer screening tool on a population wide scale. It may have a role in symptomatic patients in whom a cancer may be a realistic possibility but for whom a rapid referral wouldn’t be an option within the NHS. Again, the risks of a false positive result still exist. A further study looking at the Galleri test in those with symptoms showed that of those with a positive test 76% had cancer and 24% turned out not to have a cancer. The specificity was high however (few false positives).
So, we wait for outcome data from the original trial and in the meantime the test is not available to purchase. It appears the holy grail of a test that we can expect to detect asymptomatic meaningful cancer at a stage early enough to improve outcomes and at the same time avoid the real fear from a false positive test is still some way off.
If you have any questions about the Galleri test, cancer screening or concerns about symptoms, please don’t hesitate to get in touch with the team on 0207 245 9333 or book an appointment online.
References:
- https://www.bmj.com/content/386/bmj.q1706#:~:text=Multicancer%20early%20detection%20tests%20such,has%20been%20set%20too%20low.
- https://www.cancertherapyadvisor.com/features/mced-testing-cancer-diagnosis-progress-roadblocks/
- https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(23)02830-1/fulltext
About the author
BA (Hons) MB BSMRCGP
“The most fulfilling part of this job is making a real difference to patients with timely diagnoses and supporting them going through a difficult time”.
I enjoy the full variety of General Practice and recognise the benefits in both continuity and time that private care affords. I have a particular interest in disease prevention, chronic disease management and paediatrics.
The key to our success is maintaining and improving our patients’ health, managing chronic disease meticulously and working alongside the best specialists when needed.
