Blood tests for cancer? We’re still a way off

A new kind of blood test promises to find cancer early – sometimes even before symptoms appear.

The pitch is compelling: a single sample of blood could scan the body for dozens of different cancers at once, catching disease at a stage when it is easier to treat and more likely to be curable. For people who fear cancer – which is most of us – this sounds like a medical revolution.

These tests look for tiny pieces of DNA from cancer cells that are circulating in the blood – something my research teams have spent years working on. In the lab, powerful machines analyse these DNA fragments, searching for patterns that suggest a hidden cancer somewhere in the body.

Instead of waiting for a lump, unexplained weight loss or other symptoms, you could have a blood test every six or 12 months to check if cancer is starting to grow. NHS England described the test – which they were trialling in 142,000 patients – as “the beginning of a revolution”.

The revolution postponed

But when researchers have put these tests through their paces, the reality has fallen well short of the headlines. In one large recent UK study, the blood test missed most cancers that participants went on to develop.

A negative test may feel like a clean bill of health, but at the moment, it is nothing of the sort. This matters because people naturally change their behaviour when they are reassured. If you believe a high-tech blood test has “ruled out” cancer, you may delay seeing a doctor when symptoms appear, or dismiss nagging changes in your body as nothing to worry about.

Traditional screening tests have their own problems, but they are built on decades of evidence. Mammograms for breast cancer, colonoscopy or stool tests for bowel cancer, and cervical screening all went through long, careful trials to show they save lives overall, not just that they find more abnormalities.

Even then, they can miss cancers, and they can also pick up growths that would never have caused harm. With multi-cancer blood tests, the evidence is much thinner, and we still do not know whether using them in healthy people actually reduces deaths from cancer.

The tests also generate false alarms. Sometimes the test can flag people as having cancer when they don’t, causing huge amounts of stress. Health systems that are already stretched risk being overwhelmed by follow-up investigations, triggered by blood test results, that may ultimately lead nowhere.

Cost is another consideration. These tests are not cheap to develop or buy. If these tests are used widely before we know whether they work, health services could waste money and staff time on unproven technology instead of on proven measures like prompt diagnosis, smoking cessation, weight management and ensuring that existing screening programmes reach the people who need them most.

Yet it would be wrong to dismiss these tests entirely. The underlying science is sophisticated, and it is advancing quickly. In high-risk groups – for example, people with strong family histories of certain cancers, or those with inherited genetic mutations – carefully used blood tests might genuinely help to detect tumours earlier than we can today.

They are also helpful in checking if cancer is returning after treatment, or in choosing treatments that match the specific biology of a person’s cancer.

The deeper issue is how we introduce such technology into everyday life. There is a long history of medicine being captivated by new treatments and procedures, only to discover later that the harms and compromises were greater than expected.

Early cancer blood tests are arriving at a time when trust in institutions is fragile, misinformation spreads fast, and many people understandably feel that getting to see a doctor at all is increasingly difficult. Adding another layer of complexity and uncertainty could easily widen inequality between those who can pay privately for extra tests and those who cannot.

Sensible steps you can take

While we wait for better evidence, there are still sensible steps people can take. First, if you are invited to take part in a trial of a new cancer blood test, ask what the study is trying to show and what is already known. Genuine clinical trials, run through the NHS or academic centres, are how we answer important questions about benefits and harms.

Second, if you are offered one of these tests privately, ask who will interpret the result, what support you will get afterwards, and whether it is likely to change your care in a meaningful way.

Most importantly, do not let the idea of a “simple blood test” overshadow the basics. If you notice a new lump, unexplained bleeding, persistent cough, weight loss or a change in bowel habit that goes on for more than a few weeks, you should still contact your GP, even if a previous test was normal.

Trust your instincts about your own body and keep pushing if you feel something is wrong. Stories of delayed diagnoses repeatedly show that persistence from patients, families and doctors can make a crucial difference.

Cancer outcomes have been improving slowly over time, thanks to a combination of better treatments, earlier diagnosis and public health measures. New technology, including blood tests that scan for traces of cancer, will probably play a part in the next chapter of that story. But on their own, they are not a magic bullet against disease.

Justin Stebbing does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.