For nearly 400 patients, tumor biopsies were available, allowing for direct comparison to the blood test results from the same patient. For certain mutations that drive tumor growth, if a particular mutation was found in the blood it was also found in the tumor 94 to 100 percent of the time.

There was much less agreement for mutations that predict resistance to particular drugs. Those might have arisen only after treatment started, so might not have been seen in the tumor biopsy, which is usually taken at the time of diagnosis.

One shortcoming of the liquid biopsy was that for about 15 percent of the patients over all, no tumor DNA was detected in the blood.

“There are simply tumors that do not shed DNA into circulation at detectable levels, so we are bound to miss them,” said Dr. Mack, who has been a paid speaker for Guardant.

Dr. Edward Kim, an expert on lung cancer mutations who was not involved in the study, said the results showed the liquid biopsy accuracy was “very good.” He said, however, that use of an actual tumor sample allows for a more thorough analysis, including more mutations than is possible with a blood sample.

“I’m not personally ready to give up tissue,” said Dr. Kim, who is chairman of solid tumor oncology at the Carolinas HealthCare System’s Levine Cancer Institute in Charlotte, N.C. “It’s still the gold standard.”

Still, he said, there are times when a tissue biopsy cannot be obtained, and it is difficult to do second and third tissue biopsies on a patient. In those cases, he said, “I love the option of having the blood test available.”