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British scientists say they’ve got a promising new blood test that might warn breast cancer patients that they’re about to have a relapse.

It’s the latest in a line of so-called liquid biopsies, which seek to find tiny tumor cells circulating in the blood long before they take hold somewhere and grow into a fresh tumor.

The idea’s not new, but it’s been harder to develop into a useful product than originally thought.

Surgery, chemotherapy and radiation usually cures breast cancer, but not always.

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The team at London’s Institute of Cancer Research and colleagues based this test on each patient’s individual tumor. They sampled breast tumors from 55 patients for their DNA, running full sequences to identify the precise genetic signature of that tumor.

Then they developed a test for each mutation—a painstaking process.

Every six months, they tested the volunteers’ blood for cells with that genetic signature.

Over two years, 15 patients had the breast cancer come back. The test caught 12 of the cases, an average of eight months before other tests detected the tumor, the researchers reported in the journal Science Translational Medicine.

But the test failed to find circulating tumor cells in the brain. That’s likely because of the blood-brain barrier, a filtering system that makes it hard to see changes in the blood circulating in the brain. It also makes it hard to get drugs into the brain.

The test is not anywhere close to being commercially developed or available for patients. In its current form, it wouldn't be easy to give and it would be costly. But it's progress, the researchers said.

"We have shown how a simple blood test has the potential to accurately predict which patients will relapse from breast cancer, much earlier than we can currently,” said Dr. Nicholas Turner, who led the study.

“We also used blood tests to build a picture of how the cancer was evolving over time, and this information could be invaluable to help doctors select the correct drugs to treat the cancer."