Blood-Based Cancer Testing: Five Things to Know
Blood-Based Cancer Testing: Five Things to Know
Blood tests designed to detect cancer are gaining popularity, but how effective are they? We ask several Stanford Medicine experts to weigh in—including Department of Health Policy Chair Douglas K. Owens.
When the Galleri test hit the market in 2021, it sounded enticing — it apparently had the ability to detect dozens of cancers with a standard blood draw.
Unlike existing medical procedures that look for a single type of cancer, it was designed to spot the DNA “fingerprint” of more than 50 cancers, including some that doctors don’t routinely screen for. The hope was that the test could detect disease early, when patients often have more treatment options and better odds of survival.
The company has reportedly sold more than 420,000 test kits at a cost of $949 each even though Galleri — and similar options that have sprung up in recent years — have not yet received approval from the U.S. Food and Drug Administration.
Most people pay out of pocket for such tests, which are generally available only by prescription. In February, Congress paved the way for Medicare to eventually cover such screening technologies, known as multicancer early detection tests.
But Maximilian Diehn, MD, PhD, a professor of radiation oncology at Stanford Medicine, is among the experts who are skeptical about these tests. “Even though they’re commercially available,” he said, “I don’t think they’re ready for prime time.”
Diehn is one of the world’s leading researchers developing ultrasensitive blood tests that detect tiny amounts of tumor DNA circulating in the bloodstream. He’s leading research on similar tests that rely on RNA.
Douglas K. Owens, MD, the Henry J. Kaiser, Jr. Professor, agrees that the tests haven’t yet proven their ability to improve outcomes for patients. He directs the Stanford Department of Health Policy and chaired the U.S. Preventive Services Task Force, which sets national guidelines for preventive care.
We asked Diehn and Owens to separate hype from facts when it comes to blood-based cancer testing. Here are five things they think patients should know.
- Blood Tests for Cancer Are Not All the Same
A routine blood test your primary doctor orders, known as a complete blood count, can’t detect or rule out most cancers. It can help identify blood cancers such as leukemia or reveal abnormalities in blood-cell production, which may be an indirect sign of the disease in some patients. “It can tell you that something is off with the blood system, but it doesn’t directly detect most types of cancer,” Diehn said.
A variety of more specialized blood tests zero in on cancer specifically. They mainly work by hunting for molecules that cancer cells make and that end up in the blood. While some focus on measuring proteins, others look for cancer cells, pieces of DNA or other markers. These tests use distinct technologies and serve different purposes. Some options search for traces of a single type of cancer, while others are designed to diagnose many. And while some are intended to detect disease early, others are for aiding treatment of patients with a cancer diagnosis.
Protein-based blood tests date back to the 1960s. They measure proteins made by cancer cells or by the body in response to cancer, such as prostate-specific antigen for prostate cancer, cancer antigen 125 for ovarian cancer and alpha-fetoprotein for liver cancer. Elevated levels of these proteins can be a sign that someone has the disease. But they can also rise for other reasons, and some people who have cancer have normal protein levels. These tests usually aren’t reliable enough to diagnose cancer on their own.
A newer class of tests look for DNA in the blood that comes from cancer cells. “These are more specific than the protein-based tests,” Diehn said. Everyone has fragments of DNA floating in their blood, known as cell-free DNA. Bits that derive from cancer cells are called circulating tumor DNA. Even small amounts of these fragments found in the blood can signal a problem. “In someone without cancer, you wouldn’t expect to find DNA carrying cancer-driven mutations,” Diehn said. “That becomes a very specific test for the presence of cancer.”
Another type of test looks not for fragments of mutated DNA, but for circulating tumor cells, or actual cancer cells floating in the blood. Tumors can shed cells into the bloodstream in patients with advanced disease. Today, the technology is mainly used in research. “Those tests have not proven to be clinically useful because in most cancer patients, there aren’t enough of the circulating tumor cells to reliably detect them,” Diehn said.
Go to Full Q&A at the Stanford Medicine News Center
