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........

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