Scientists are working on a blood test that may catch five common cancers years sooner than current methods.
The blood test, which is still experimental, hunts for certain genetic "signatures" associated with tumors. Researchers found that it can detect five types of cancer -- colon, esophageal, liver, lung and stomach -- up to four years earlier, compared to routine medical care.
More research is needed to confirm the test's accuracy. But these initial results "offer hope," said researcher Kun Zhang, a professor of bioengineering at the University of California, San Diego.
Having a "convenient and quick" blood test that can catch cancers earlier could give people more treatment options, and hopefully improve their survival odds, Zhang said.
In fact, such a test is the proverbial "holy grail" in cancer research, said Dr. William Cance, chief medical and scientific officer of the American Cancer Society.
A number of experimental blood tests have been studied and are under development. But the new study, Cance said, is an "important step forward."
"This test has one of the highest sensitivity rates that's been reported, and it's able to do it with a relatively small blood sample," he said.
Sensitivity refers to a test's ability to accurately pick up all people with a disease. In this study, the blood test had a sensitivity rate of about 95% in people free of cancer symptoms.
However, both Zhang and Cance stressed that there's a long road ahead.
As a next step, the test performance should be validated in additional study groups, according to Cance.
And ultimately, Zhang said, any blood test for cancer screening would have to be proven in a clinical trial.
The concept of using a blood test to detect early-stage cancers is based on a simple fact: Tumor cells regularly shed bits of their DNA into a person's bloodstream.
A snag, however, is that the DNA is present in only small amounts -- especially when tumors are small. That can make it hard to pick up, according to Zhang.
The blood test his team developed looks not for DNA, but for chemical changes to DNA called methylation. Methylation normally controls which genes are turned "on" or "off," Cance explained. Abnormal methylation patterns can signal the presence of cancer.
To gauge the test's performance, Zhang's team used blood samples from 605 people who were part of a long-term health study. At the outset, all were healthy and free of cancer symptoms; but over the next four years, 191 were diagnosed with colorectal, esophageal, liver, lung or stomach cancer.
Overall, the blood test detected cancer in 95% of those individuals -- up to four years before their actual diagnosis.
"There were already signatures in their blood that showed they had cancer," Zhang said.
The accuracy did vary by cancer type, he added. The test caught all cases of liver cancer, for instance, and 91% of esophageal cancers. The overall rate of false-positives was around 5%.
Ideally, Cance said, "you want a false-positive rate as close to zero as possible."
That's to limit complicated scenarios where, for example, a blood test says cancer is present, but a follow-up mammogram shows no tumor -- leaving uncertainty about which is right.
As is, the current test only indicates whether any of five cancers is present -- and not which type. Further refinement, Zhang said, might allow testing to be more precise about the tumor location.
But any future blood test, Cance said, would only be an initial screening: It would have to be followed up by more extensive tests to get a definite diagnosis.
Earlier diagnosis of a cancer does not necessarily mean better survival odds. But, Cance noted, the five cancers detected by this test are all treatable when found early.
Zhang co-founded Singlera Genomics, the company developing the blood test.
The American Cancer Society has more on cancer screening.