Blood samples that could potentially be used to screen for cancer

Can a Blood Test Find Cancer Early?

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What if there were a blood test that anyone could get as part of a routine physical that could detect cancer before any symptoms appeared — when presumably it could be treated most effectively?

Finding a blood test that can do that, and truly save lives as a result, is sometimes referred to as the “holy grail” of cancer research.

There’s lots of research ongoing with that as the ultimate goal. But are we there yet?

One company, which actually calls itself GRAIL, is already marketing a test called Galleri that is supposed to detect signs of cancer through a screening blood test.

The Galleri Test

The Galleri test starts with a simple blood draw that can be taken in a regular visit with a health care provider. The test can detect signs that certain types of cancer may be present. The test cannot make a diagnosis of cancer, but only suggest that additional testing is needed to determine whether cancer is or is not present.

The test is marketed as being for anyone with “an elevated risk for cancer, such as those aged 50 or older.” It is intended to be used in addition to, rather than replacing, other cancer screening tests such as mammograms and colonoscopies.

Galleri has been evaluated in a “case-control” clinical trial, where some of the participants were known to have cancer and others did not, to determine how well the test did at signaling when cancer was or was not present. The study’s findings were published in the Annals of Oncology in 2021.

Based on the findings from that case-control study, Galleri is now being marketed by GRAIL and some physicians offer it. It has not been approved by the FDA.

Benefits and Costs

How well does the Galleri test do what it’s intended to do–identify cancer early when presumably it’s most treatable? In the case-control clinical study, the test correctly flagged cases where stage I-IIII cancers were present 67% of the time. It identified only 17% of stage I cancers.

The test was able to detect cancer “signals” across a range of 50 cancer types, including cancers that are otherwise difficult to find at early stages, such ovarian cancer and pancreatic cancer. The test was found to be accurate about 88% of the time in identifying the body site where cancer was occurring.

On the other hand, what are the chances the test could incorrectly say you have signs of cancer being present–a “false positive”?

The study results suggested that in a large screening population only 0.5% would receive a false positive result. While that seems good, in a large population that could still be a lot of people getting a false positive result.

If you receive a positive result from a Galleri test, what is the likelihood that you actually have cancer? Based on the case-control study results, the researchers estimated that a positive finding from Galleri will lead to a confirmed cancer diagnosis less than half of the time (about 56% will be false positives).

The test could turn out to be beneficial for someone who has an early-stage cancer that is not currently detectable through other screening methods, and having been found “early”, can be treated effectively. This is the hoped-for scenario, that the test could benefit people in this way.

But if the result is a “false positive”, the cost can be high in terms of unnecessary tests such as scans and biopsies.

And there’s the financial cost. The test is priced at $949 and the charge must be paid out of pocket as insurance does not cover it. In addition, follow-up scans and other tests also may not be covered by insurance.

Ongoing Studies

The Galleri test needs to be evaluated further to find out how well it performs when it’s actually used as a screening test. Several clinical trials are underway to make this assessment. These trials will provide more complete information about the test’s benefits and limitations.

Others are developing cancer screening blood tests as well. For example, a study reported in 2020 evaluated the safety and feasibility of a test called CancerSEEK that also looks for signs of cancer in a simple blood draw. That test is not yet on the market and additional trials are planned or in process.

The most important question of course is whether cancer screening blood tests actually save lives. Do people live longer when their cancers are found in this way or do they just know they have cancer for longer than they otherwise would have? Some of the ongoing clinical trials will be looking to answer this critical question.

Additional Reading

This informative article by Piri Welcsh, PhD on the website of FORCE, a nonprofit that advocates for the heredity cancer community, discusses cancer screening blood tests, the Galleri test and issues that need to be considered.

Also worth reading is this recent article by Gina Kolada in the New York Times which provides additional perspectives on the risks involved in the use of cancer screening blood tests.

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