The Centers for Disease Control and Prevention (CDC) initially declined to test a California patient for coronavirus because of narrow testing criteria, delaying the identification of a new possibly pivotal case, according to officials at the hospital treating the patient.

UC Davis Medical Center wrote in a memo to staff that the patient was transferred from another hospital on Feb. 19 with a suspected viral infection. The hospital requested coronavirus testing, but the CDC initially declined because the patient, who had not recently traveled to countries with outbreaks or been in contact with someone with the virus, did not meet the testing criteria.

It was not until Sunday that the CDC agreed to do a test, and the results then came back on Wednesday as positive.

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Officials said the case could mark a turning point as the first case of the virus spreading among the general public in the United States, because there is no known origin for it.

“Upon admission, our team asked public health officials if this case could be COVID-19,” the UC Davis Medical Center said in a statement, referring to coronavirus. “We requested COVID-19 testing by the CDC, since neither Sacramento County nor [California Department of Public Health] is doing testing for coronavirus at this time.”

“Since the patient did not fit the existing CDC criteria for COVID-19, a test was not immediately administered,” the statement added. “UC Davis Health does not control the testing process.”

In a statement on Thursday afternoon, the CDC said it is concerned by the reports but said their records showed the CDC only learned of the case on Sunday, and requested specimens for testing that same day.

"CDC is concerned about reports that testing for COVID-19 for the California patient announced on February 26 was delayed as a result of CDC," an agency spokesman said. "We are investigating this carefully, however, a preliminary review of CDC records indicates that CDC was first informed about this case on Sunday, February 23. That same day CDC requested specimens from the patient to test for COVID-19 exposure."

The CDC also noted that testing guidelines currently allow testing for people who do not meet the formal criteria but whom doctors suspect could have the coronavirus.

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Experts raised concern that the CDC's narrow testing criteria could mean that there are more cases circulating among the general public that have not been identified.

“This raises the possibility that we only think we have few cases in the US, because we have only tested a few hundred people,” tweeted Ronald Klain, who was the czar overseeing Ebola response in the Obama administration.

The California hospital offered reassurances about the risk of spread of the virus from the patient, saying that “because of the precautions we have had in place since this patient’s arrival, we believe there has been minimal potential for exposure here at UC Davis Medical Center.”

A “small number of employees” are being asked to stay home and monitor their temperature “out of an abundance of caution,” the hospital said.

“With testing so limited right now, we're flying somewhat blind, while being reassured that we're not flying toward a mountain,” tweeted Jeremy Konyndyk, a senior fellow at the Center for Global Development who directed the U.S. Agency for International Development's Office of Foreign Disaster Assistance during the Ebola outbreak. “I'd be a lot more confident in that message if the pilot could see out the windscreen.”

Updated: 5:12 p.m.