Multi-cancer blood test shows 66% accuracy in NHS trial, could accelerate diagnosis

A simple blood test that hunts for fragments of tumour DNA circulating in the bloodstream has shown it can identify cancer in roughly two out of every three people who walk into their GP surgery with warning signs. The Galleri test, developed by California-based Grail, screened 6,238 patients across England and Wales who had already reported symptoms to their doctors—weight loss, anaemia, abdominal pain, the kinds of complaints that could point toward cancer or toward a dozen other things entirely. Of those patients, 323 tested positive. When doctors followed up, 244 of them were confirmed to have cancer. That 66% accuracy rate, presented this week at the American Society of Clinical Oncology conference in Chicago and published in The Lancet Oncology, represents the first substantial evidence that a multi-cancer blood test can work inside the NHS system.

But the number masks a more complicated picture. The test's reliability depends entirely on how far the cancer has already spread. For tumours caught at stage one—the earliest point—the test found only 24% of them. By stage four, when disease has metastasized throughout the body, accuracy climbed to 95%. Bowel cancer accounted for 37% of the diagnoses the test identified, followed by lung at 22%, uterine at 8%, oesophago-gastric at 6%, and ovarian at 4%. The test can also pinpoint where in the body the cancer originates with 85% accuracy, a detail that could spare patients from some of the invasive investigations doctors currently use to narrow down a diagnosis.

The appeal is straightforward: most people with cancer first see their primary care doctor, not an oncologist. They arrive with non-specific complaints that could mean almost anything. A GP faces a diagnostic puzzle—pursue cancer workup, or investigate other causes? The test offers a tool to answer that question faster. Brian Nicholson, an associate professor at Oxford's Nuffield Department of Primary Care Health Sciences, framed it this way: the test can confirm whether a symptomatic patient warrants cancer evaluation before doctors send them down other diagnostic paths, potentially avoiding costly and invasive procedures that might ultimately prove unnecessary.

The NHS is already running a second trial to test whether the Galleri test can detect cancers in people with no symptoms at all—a different and potentially more valuable application. Results are expected later this year. Peter Johnson, the NHS national director for cancer, called the current findings "the first step in testing a new way to identify cancer as quickly as possible." He emphasized that earlier detection is vital, and that this test could help catch more cancers at earlier stages, when treatment is most likely to succeed.

Yet experts are cautious about what the data actually shows. The test does not catch all cancers, and it is not expected to replace existing screening programmes for breast, cervical, or bowel cancer—programmes that have already proven their worth. Lawrence Young, a professor of molecular oncology at the University of Warwick, noted that more research is needed before doctors can confidently trust a negative result. "To really trust that a negative result on blood testing means no cancer will require more studies," he said. Dr Richard Lee, a consultant physician at the Royal Marsden Hospital and team leader for early diagnosis at The Institute of Cancer Research in London, agreed: the test remains a research tool, not ready for routine clinical use, but could become "a very important tool for cancer diagnosis in the future." The pathway from promising trial data to standard practice in the NHS is long, and this test has only just begun to walk it.