Limited testing has blinded Americans to the scale of the outbreak so far, impeding the nation’s ability to fight the virus through isolating the sick and their contacts, public-health officials say. As of early Wednesday, about 6,500 people in the U.S. had tested positive, data compiled by Johns Hopkins University show, but the Centers for Disease Control and Prevention had reported only about 32,000 tests conducted at its facilities and other public-health labs. Limited testing is also keeping patients like Justin LaBor in the dark, despite recent improvements. Mr. LaBor, 36 years old, said he went to the emergency room at AtlantiCare Regional Medical Center in Pomona, N.J., Monday with a fever and dry cough, symptoms typical in a coronavirus infection. Doctors admitted him, but he hasn’t been tested for the virus, he said Tuesday, gasping for breath over the phone.

“Everyone just told me there were no tests and I didn’t check all the boxes for the state,” said Mr. LaBor, a social-media marketer from Elwood, N.J., referring to the criteria that state labs require patients to meet before running tests. A New Jersey health-department spokeswoman said the state “has sufficient lab capacity to test those who meet the testing criteria.” An AtlantiCare spokeswoman said the hospital system “is not testing patients in a widespread manner for coronavirus.”

While the virus was quietly spreading within the U.S., the CDC had told state and local officials its “testing capacity is more than adequate to meet current testing demands,” according to a Feb. 26 agency email viewed by The Wall Street Journal, part of a cache of agency communications reviewed by the Journal that sheds light on the early response. The agency’s data show it tested fewer than 100 patients that day.

When the CDC first dispersed test kits in early February, it shipped them to a network of state and local government labs and restricted testing to people with virus symptoms who had recently traveled to China, where the virus first emerged, or had been exposed to a known case. Federal officials hoped the virus could be contained—even as they disputed alarms from those on the front lines that the CDC’s guidelines weren’t keeping up with the outbreak’s spread, emails between the U.S. agency and local officials show. The government left other laboratories on the sidelines for crucial weeks.