A new blood test for dozens of cancers promises earlier answers — but raises big questions
The headlines
- Advertisement -
Last week’s presentation at the European Society for Medical Oncology in Berlin put a new-generation blood test back in the headlines: Galleri, a so‑called multi‑cancer early detection (MCED) assay developed by Grail, correctly identified a cancer signal and the likely tissue of origin in a large real‑world cohort and — crucially — picked up many tumours at an early, more treatable stage.
The Pathfinder 2 study followed 23,161 symptom‑free adults in the United States and Canada for at least a year. The test flagged a cancer signal in 216 people; 133 of them were later diagnosed with cancer — a positive predictive value of about 62% among those with a positive test. Galleri’s ability to rule out disease in those without cancer was reported as 99.6%, and in 92% of cancer cases the test identified the likely organ or tissue where the tumour began. More than half of the cancers detected were stage I or II.
What those numbers mean
At first glance the results are arresting: a test that can screen for more than 50 cancer types from a single tube of blood, detect many tumours early and point clinicians toward the likely site of disease could be transformative — particularly for cancers that currently have no routine screening tests, like pancreatic and ovarian cancers.
But these figures need interpretation. The test’s sensitivity — the share of true cancers it flagged when it was positive — was reported at about 62% in people where Galleri thought cancer might be present. That means roughly four in ten cancers that produce a detectable signal may still be missed. The near‑perfect specificity (99.6%) signals a low rate of false positives across the whole screened population, but the practical impact of even small numbers of false alarms can be large when scaled to millions of people.
Grail’s Harpal Kumar, president of International Business and BioPharma and a former head of Cancer Research UK, called the findings “very exciting,” saying the test added “seven times as many cancers” as existing screening programmes in the study population. He also stressed the efficiency gains from the test’s tissue‑of‑origin prediction: “If we can help clinicians direct those investigations, then we can make much better use of what is very scarce diagnostic capacity,” Kumar said.
Promise and pitfalls
There are reasons for both enthusiasm and caution. The potential upside is clear: many cancers are diagnosed only after symptoms appear, by which time curative options may be limited. A blood test that reliably shifts diagnosis to earlier stages could reduce mortality and the human cost of late disease — a public‑health win on a global scale.
But early detection carries trade‑offs. One is overdiagnosis — finding cancers that would not have progressed in a person’s lifetime — and the cascade of tests, biopsies and anxiety that can follow. Another is the risk of false reassurance; a negative test does not mean “no cancer,” especially given the test’s imperfect sensitivity. There are also practical constraints: imaging machines, specialist clinics and biopsy services are already stretched in many health systems. Directing thousands more patients into diagnostic pathways would require investment and careful planning.
Modelling published in BMJ Open earlier this year suggested annual MCED testing from age 50 could reduce late‑stage diagnoses by nearly half and cut deaths by about a fifth within five years compared with usual care. Models are useful but not definitive. What clinicians and policymakers really want is evidence that screening reduces deaths or improves quality of life in randomised trials — not only that more cancers are found earlier.
A global lens: unequal access, rising burden
Cancer is a growing global problem. As populations age and lifestyles change, the number of new cases worldwide continues to climb — the International Agency for Research on Cancer estimated close to 19.3 million new cases in 2020. Yet the benefits of any new screening technology often flow unevenly. High‑income countries with robust diagnostic infrastructure could adopt MCED tests faster, while low‑ and middle‑income countries — where cancer mortality is already highest — may be left behind unless cost and implementation are addressed.
The test’s cost, who pays, and whether it will replace or simply add to existing screening are unresolved questions. There are also regulatory and ethical dimensions: how to counsel people about uncertain results, how to monitor downstream harms, and how to ensure communities that have long distrusted healthcare systems are not exposed to greater burdens.
Where the evidence needs to go next
The Pathfinder 2 results are an important step, but they are not the final chapter. We need randomised controlled trials with mortality or patient‑centred endpoints to demonstrate that routine MCED screening improves outcomes compared with current practice. We also need independent cost‑effectiveness analyses and real‑world studies that examine harms, stage migration, and health‑system impacts.
For clinicians, the test may offer a new tool for patients with vague symptoms — a way to triage scarce diagnostic capacity and focus investigations. For patients, it promises answers at a time when uncertainty can be crushing. For health systems, it presents both opportunity and obligation: to weigh potential lives saved against the risks of overdiagnosis, to make hard choices about allocation, and to ensure equitable access.
As the technology matures, the fundamental question remains not only whether we can detect more cancers earlier, but whether doing so will save lives without creating new harms. When a fingertip of hope — and a single vial of blood — can open so many doors, who should walk through them, and on what terms?
By Abdiwahab Ahmed
Axadle Times international–Monitoring.
