With 44 million Medicare beneficiaries and an annual test costing about $1,000 a year plus expensive scans and biopsies for those whose tests are positive, the price tag could be substantial.

He and other critics warn that the risks of unleashing the tests are substantial. Paradoxical as it may sound, finding cancers earlier could mean just as many deaths, with the same timing as without early diagnosis. That is because — at least with current treatments — cancers destined to kill are not necessarily cured if found early.

And there are other risks. For example, some will have a positive test, but doctors will be unable to locate the cancer. Others will be treated aggressively with surgery or chemotherapy for cancers that, if left alone, would not have grown and spread and may even have gone away.

Dr. Beer acknowledges that a cancer blood test “doesn’t come without risks or costs, and it is not going to detect every cancer.”

But, he said, “I think there’s promise for a real impact.”

Others experts are worried.

Dr. Barnett Kramer, a member of the Lisa Schwartz Foundation for Truth in Medicine and former director of the Division of Cancer Prevention at the National Cancer Institute, fears that the tests will come into widespread use without ever showing they are beneficial. Once that happens, he said, “it is difficult to unring the bell.”

“I hope we are not halfway through a nightmare,” Dr. Kramer said.

The Damocles Syndrome

When Susan Iorio Bell, 73, a nurse who lives in Forty Fort, Pa., saw an ad on Facebook recruiting women her age for a study of a cancer blood test, she immediately signed up. It fit with her advocacy for preventive medicine and her belief in clinical trials.