“A false negative is problematic because it tells the patient they don’t have the virus,” said the doctor, Craig Deligdish, an oncologist at Omni Healthcare, a Melbourne, Fla., medical group. Meanwhile, that patient can unwittingly spread it, he said.

Health experts say they now believe nearly one in three patients who are infected are nevertheless getting a negative test result. They caution that only limited data is available, and their estimates are based on their own experience in the absence of hard science.

That picture is troubling, many doctors say, as it casts doubt on the reliability of a wave of new tests developed by manufacturers, lab companies and the U.S. Centers for Disease Control and Prevention. Most of these are operating with minimal regulatory oversight and little time to do robust studies amid a desperate call for wider testing.