Researchers are getting closer to developing a blood test to detect cancer. But doctors say there’s a long way to go.

Every year, more than half a million Americans die of cancer.

Earlier diagnosis and treatment could save many of those lives.

But early stage cancer doesn’t always produce symptoms. There are screening tests for some types of cancer, but not for most.

Scientists at Johns Hopkins Sidney Kimmel Comprehensive Cancer Center are trying to change that.

The researchers have developed a blood test that may be a stepping-stone to earlier diagnosis.

The goal is to have a test precise enough to detect cancer without producing a lot of false-positive results.

The test can tell the difference between cancer and noncancer DNA in the blood. That includes altered DNA that could otherwise be mistaken for cancer biomarkers.

The full report was published in Science Translational Medicine.

Doctors currently use blood tests, or “liquid biopsies,” to monitor people with cancer during treatment. This is possible because biopsied tumors can be compared with DNA circulating in the blood.

But to develop a screening test for use in apparently healthy people, it must be able to find DNA alterations that haven’t already been identified.

It must also be able to distinguish between cancer-derived mutations and normal, inherited DNA variations.

All cells shed DNA into the circulatory system (cell-free DNA, or cfDNA). People with cancer tend to have higher levels of cfDNA in their blood. Cancerous tumors also shed DNA into the blood (circulating tumor DNA, or ctDNA).

The Johns Hopkins team developed a method called targeted error correction sequencing (TEC-Seq). It can detect even very low amounts of ctDNA, which would most likely be found in people who have early-stage cancer.

The researchers took blood samples from 200 people with different stages of breast, lung, ovarian, and colorectal cancer.

The test was able to identify 62 percent of stages 1 and 2 cancers.

There were no false-positives among 44 healthy people who were also tested.

“This study shows that identifying cancer early using DNA changes in the blood is feasible, and that our high accuracy sequencing method is a promising approach to achieve this goal,” Dr. Victor Velculescu, professor of oncology at the Johns Hopkins Kimmel Cancer Center said in a press release.

In theory, an accurate blood test to screen for cancer would promote early detection and lifesaving treatment before cancer has a chance to spread. It would also cut down on false positives that lead to more invasive testing and unnecessary treatment.

Dr. Lawrence Wagman, surgical oncologist and executive medical director at St. Joseph’s Center for Cancer Prevention and Treatment in California, spoke with Healthline about the implications of this research.

“Right now, we have cancers that are very high risk. We have very few tests to screen for them. Ovarian cancer and pancreatic cancer usually present late and have high death rates,” he said.

“You plug something like this into a high-risk population and do it on a regular basis, and the application of this kind of test could be phenomenal,” said Wagman.

He said that although blood tests require a needle, most people would consider them noninvasive compared with other procedures used to diagnose cancer, such as biopsy.

Wagman noted that such a screening test would likely be much less expensive than some of the current cancer screening methods.

He also cautioned that these benefits have yet to be proven.

There’s already some controversy over whether doctors are overtreating cancers that would never become a problem, such as slow-growing forms of breast and prostate cancer.

Wagman said it’s an issue that should be considered.

“It may be that many people get small cancers that go away without treatment. We’ve never been able to study this because we don’t see them when they’re that small. What if a blood test indicated breast cancer, but it’s so small that we couldn’t find it on mammogram, MRI, or PET scan? What would we do with that information? We could be finding things too soon,” he said.

“An 80-year-old man with early-stage prostate cancer with no symptoms wouldn’t require treatment. The blood test would be abnormal. But consider how it would feel if someone told you a blood test proves you have cancer, but that we shouldn’t do anything about it,” said Wagman.

“It would be a great test, but application is very critical,” he said. “It may eventually find a place in diagnosing certain cancers.”

Wagman explained that right now this blood test has no broad application to healthy patients.

He suggested that the focus should be on high-risk groups, such as those with a genetic risk — BRCA gene mutation carriers, for example, coupled with disease that has poor outcome.

The researchers included smokers as a group who would benefit from earlier detection.

The Johns Hopkins researchers say they need to validate their study in much larger numbers of people.

Wagman agrees.

“The application of tests like this looks different when applied to an unknown population of thousands of people rather than a steady population that is well controlled,” he said.

“The first phase is to see how much it works in a particular population and from one country to another. A lot depends on a person’s background. It’s a great idea to be looking at something that’s just a blood test. And it’s in line with a lot of things we’re doing now. It’s very interesting, but now let’s see how it will work,” he continued.

There won’t be a blood test to screen for cancer in the general population any time soon. But the research still holds promise.

“There’s a very long way to go,” said Wagman. “But this might drive another researcher to look at the same methodology and improve upon it.”