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Researchers say they have taken a big step towards developing a test that can tell people if they have cancer long before the first symptoms show up.

The blood test detected the majority of cancers in people with four of the biggest cancer killers: breast, colon, lung and ovarian cancer, the team at Johns Hopkins University said.

Scientists have discovered that dying tumor cells release small pieces of their DNA into the bloodstream. These pieces are called cell-free circulating tumor DNA (ctDNA). National cancer Institute

The test is a long way from being used to screen for cancer, but the study shows a way to get there, the team reported in the journal Science Translational Medicine.

“There is a lot of excitement about liquid biopsies, but most of that has been in late-stage cancer or in individuals where you already know what to look for,” said Dr. Victor Velculescu, professor of oncology and pathology at the Johns Hopkins University Kimmel Cancer Center.

“The surprising result is that we can find a high fraction of early-stage patients having alterations in their blood,” said Velculescu, who led the study team.

It was not a slam dunk, but the test found cancer in the blood of more than half the patients who had been diagnosed with stage 1 cancer. It was even more accurate in finding late-stage cancers, but the goal would be to catch cancer in its earliest, easiest-to-treat stage.

There were no false positives in 44 people who did not have cancer, they said. That's important, said Dr. Wyndham Wilson of the National Cancer Institute, because there is no point detecting cancer in people if the cancer is not going to actually cause trouble.

"You don’t want to go screening people for hallmark (cancer) mutations unless you absolutely know that when you find it, that there is a tumor there and that it is a tumor that needs to be treated," said Wyndham, who was not involved in the study.

Sometimes, early-stage tumors or precancerous growths just go away -- attacked by the immune system or because they don't thrive for other reasons.

Several different liquid biopsies are already on the market, used to help track whether cancer treatments are working. But there’s nothing yet that can detect cancer in someone who has not yet been diagnosed.

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Related: Company Bets on Catching Early Cancer With Liquid Biopsy

It’s easy to find tumor mutations if you know what to look for. “The challenge was to develop a blood test that could predict the probable presence of cancer without knowing the genetic mutations present in a person’s tumor,” Velculescu said.

Velculescu’s team developed an approach called targeted error correction sequencing (TEC-Seq for short).

“We have used this approach to examine 58 cancer-related genes,” the team wrote in their report. The method involved deep sequencing – sequencing DNA 30,000 times over to look for mutations in DNA from tumor cells that floats in the blood.

Related: Liquid Biopsy Helps Doctors Pick the Best Cancer Drug

Cancer patients had more of this DNA in their blood, the team found.

They identified 62 percent of the patients with stage I cancer – four out of eight colon cancer patients, and 90 percent of colon cancer patients with stage II, III or IV disease.

They got a positive in 45 percent of the lung cancer patients with stage I disease, 67 percent of ovarian cancer patients with stage I disease and 67 percent of breast cancer patients with stage I disease.

While that's good, it's not a great result. The test still missed a large percentage of cancers and will need much improvement, Velculescu said.

It will also have to be tried in larger groups of patients, and patients with different cancers. The first goal would be to try it in people at high risk of cancer but no symptoms yet – such as smokers, or people with cancer-causing gene mutations like BRCA mutations, Velculescu said.

Wyndham said it will be important to study such tests in large groups of people who have not had cancer diagnosed, to see if it can truly be used to screen asymptomatic people for cancer. And then it will have to be shown that using the test allows doctors to intervene sooner and help people.

Catching cancer in its earliest stages could save many lives. Cancer is the No. 2 killer overall in the United States.

”The survival difference between late stage and early stage disease in these cancers would account for more than a million lives each year worldwide,” Velculescu said.

The genetic sequencing is also expensive right now – on the order of several thousand dollars for the 30,000 repeats the team did. But costs are coming down steadily, he said.

Related: Can This Blood Test Detect Cancer?

Velculescu said Johns Hopkins had patented this test, and Velculescu is himself the founder of a company that does liquid biopsies for advanced cancer patients called Personal Genome Diagnostics.

The U.S. Food and Drug Administration has been very skeptical of blood tests that claim to diagnose disease before people have symptoms.

The agency has chastised a company called Pathway Genomics over its "liquid biopsy" test , saying the company had not shown the $699 test worked as advertised.

It’s also warned other gene testing companies but finally gave 23andMe the go-ahead to market its home DNA test – which does not include any cancer screening – in April.

Cancer screening is still often a tricky issue.

Currently, colon cancer can be detected very early with colonoscopies, and even stopped before pre-cancerous growths get out of control. But colonoscopies are uncomfortable and carry a small risk of injury.

Mammograms can detect early breast cancer, Pap smears can detect cervical cancer or pre-cancerous changes and a type of specialized chest x-ray called a spiral CT can detect lung cancer. There’s a debate over the usefulness of screening for prostate cancer but blood tests and physical exams can indicate some men at high risk.