The opinion was provided Monday in the case of Jahi McMath by Stanford University pediatric neurologist Paul Fisher.
Jahi was declared brain dead on Dec. 12 after she went into cardiac arrest following surgery to treat sleep apnea.
Her family wants Alameda County Superior Court Judge Evelio Grillo to issue what would be an unprecedented order declaring her to be alive after being declared brain dead.
A hearing had been scheduled for Thursday, but it has been postponed as the attorney representing McMath's mother seeks time to respond to Fisher's finding.
"There wasn't time to react to this letter," attorney Chris Dolan said Wednesday night. "Given the fact that we were up against the time crunch ... I thought that we were going to be in essence sandbagged."
Dolan objected to Fisher's appointment as an independent expert, arguing that the doctor has a conflict of interest because he was one of the physicians who agreed with the brain-death diagnosis in December.
Thursday's hearing wasn't immediately rescheduled, and Dolan has asked for a four-week delay so his experts and Fisher can discuss the results and come up with a plan for more tests that would satisfy them all.
"I just wanted to give these doctors a chance to talk," Dolan said.
Five other medical professionals who performed new tests on the teenager in New Jersey last month said the girl showed signs of brain function.
In his letter to the judge, Fisher replied that the tests either were irrelevant to determining brain death in a child or not carried out in accordance with accepted medical standards.
"None of the declarations provide evidence that Jahi McMath is not brain dead," he wrote.
Jahi has been kept on a ventilator and feeding tubes since she suffered severe complications from the surgery. Fisher examined her then at the request of the judge while her family fought a hospital's decision to remove the equipment.
Fisher was one of three doctors who declared her brain-dead after finding no neurological activity.
Dolan has given the judge the results of a Sept. 1 electroencephalogram that a researcher at a medical school in Cuba said showed electrical activity in Jahi's brain.
Fisher, in his letter to the judge, said the new test was performed in an apartment, not a health care setting, and the recorded activity could have come from elsewhere in the girl's body or even the environment. Regardless, he said, a flat reading on the exam is not a prerequisite for brain death.
Fisher similarly took issue with the brain scan that allegedly showed blood flowing to Jahi's brain. He said the test doctors and researchers from the non-profit International Brain Research Foundation used was incorrect and would not have demonstrated such blood flow.
Fisher also did not attach much significance to videos showing the girl moving her hands and feet in apparent response to her mother's commands.
Jahi's mother, Latasha Winkfield, has worked to keep her daughter's organs functioning on life support, first at Children's Hospital in Oakland and later at an undisclosed medical facility, and now a house in New Jersey.
Unlike California, New Jersey law allows families to reject a declaration of brain death on religious grounds and allows brain-dead patients to remain connected to ventilators.