Blood tests for cancer screening: should I get one?

You may have seen print or television ads promoting the Galleri blood test for “early cancer detection.” Neither this test nor any others are covered by Medicare or commercial insurance, but you can have it if ordered by your doctor and you are willing to pay $949 out of pocket.

Assuming you can afford it, is it worth it?

It is considered self-evident that detecting cancer early is better than finding it late, and routine colonoscopy for the detection of early cancer or pre-cancerous polyps does prevent colon cancer deaths. Screening smokers for lung cancer with low-dose chest CT scans has also been shown to reduce the death rate from lung cancer. While somewhat controversial, there is evidence that routine mammograms reduce breast cancer deaths.

This leaves many cancers for which there is no accepted screening tool. Also, not all eligible people are able or willing to have the tests mentioned. Enter the “liquid biopsy,” a single blood test that promises to detect many cancers.

These tests work by detecting minute amounts of tumor DNA or abnormal proteins secreted by cancers.

There are many problems with these tests.

First, while they are pretty good (90%) at detecting advanced cancer, which has already spread, they are much less effective (15%) at detecting early localized cancers, the kind where early detection and surgery is expected to be life-saving.

Secondly, they have a high number of “false positives:” an abnormal test but no detectable cancer. In two large studies, enrolling over 16,000 people, 600 people had positive tests. The large majority had no cancer and only 14 had early-stage solid tumors that were treated. Most of the positive tests required extensive imaging and invasive biopsies to be sure there was no cancer.

To date, there has not been any evidence that screening with a blood test saves lives.

Bottom line: have your colonoscopy. Have your Pap and mammogram. If you are middle-aged and have smoked, have a screening chest CT.

I am sure that more precise and actionable tests are on the horizon. For the time being, do not spend your money on a blood test that is unlikely to impact your survival and that may lead to uncomfortable and possibly dangerous testing.




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