Finding the perfect early screening test for cancer remains a persistent challenge for the health industry. But new findings suggest that blood tests still being assessed are showing promising results when it comes to detecting cancers at earlier stages.
Multi-cancer early detection tests are blood tests that look for DNA fragments that cancers can release into the bloodstream. The tests are abbreviated as MCED.
“These blood tests are looking for multiple different types of cancer in the blood at the same time,” said Dr. Rebecca Kaltman, medical director of the Inova Saville Cancer Screening and Prevention Center at the Inova Schar Cancer Institute in Fairfax, Virginia.
Only five tests designed to detect early stage cancer have been approved by the U.S. Preventative Task Force: mammograms for breast cancer, colonoscopies for colon cancer, Pap smears for cervical cancer, prostate-specific antigens, or PSA tests for prostate cancer, and low-dose CT scans for lung cancer.
As the other 100 or so cancers — including pancreatic, liver, ovarian and others — don’t have reliable scanning tests, they are often found by the time they have spread and are more difficult to treat.
Studies show approximately 70% of cancer deaths result from types of cancer that do not have an early screening test.
More than 800 participants from Schar were enrolled in the national Pathfinder II study of the Galleri test, which is made by Grail.
A similar MCED test, Cancerguard, is also seeking approval from the U.S. Food and Drug Administration. That test is made by Exact Sciences.
Are the tests accurate? What about false positives?
When a patient has an MCED test, “It will come back as either positive or negative, and if it’s positive it will say a cancer signal was detected,” Kaltman said.
The Galleri test generally suggests one or two sites from which the cancer may be originating, which will guide further follow-up testing to determine whether a positive finding is accurate.
As with other early screening methods, a positive result does not mean a person has cancer.
“When a mammogram is done and there’s concern about it, the vast majority of the time that concern ends up not being cancer,” Kaltman said. “About 15 to 30% of time, depending on the literature that you see, it will be cancer — so that’s what we call the positive predictive value of the test.”
So far, in clinical trials, “Galleri had a 62% chance of when it came back positive, that it was actually diagnosing a cancer,” Kaltman said.
On the other hand, if an MCED does come back negative, that doesn’t mean a person doesn’t have cancer.
“It’s very important that anybody who has this test undergoes routine screening — colonoscopy, age-appropriate mammography, et cetera,” she said.
Future clinical trials and research, likely before FDA approval, will look at the effects of test errors, as well as whether MCEDs actually help patients live longer.
“If you have a false positive, or something comes back positive that we can’t clarify, that can cause a lot of anxiety,” Kaltman said.
For the FDA-approved prostate cancer test, “We were picking up a lot of cancers that ended up probably not impacting somebody’s longevity. And individuals were subjected to invasive surgery with complications that they might not have had to go through, because it wasn’t going to impact their longevity.”
Kaltman said current clinical studies are evaluating how valuable MCEDs are in extending people’s lives.
“Are we catching cancers that we need to catch early — the ones that are aggressive and need to be detected early, to have better outcomes,” she said.