The latest confirmed coronavirus patient in the United States was mistakenly discharged from a San Diego hospital because of a laboratory error, the Centers for Disease Control and Prevention said Tuesday.

The patient was among three evacuees from China who were placed in isolation and underwent testing at the UC San Diego Medical Center last week after showing signs of the novel coronavirus.

An initial round of tests came back negative, and the people returned to a quarantine site at Marine Corps Air Station Miramar. However, further testing revealed that one of them was infected.

A CDC spokeswoman, Kristen Nordlund, said Tuesday that blood samples from the three patients were inadvertently omitted from a large test run because they were incorrectly labeled.

“CDC conveyed an informal ‘all negative’ notification for the specimens that were tested. Once labeling issues were addressed, CDC subsequently tested the three specimens that had been omitted and found that one of those three tested positive,” Nordlund told The Washington Post in an emailed statement.

“This information was conveyed to local public health and CDC teams immediately,” she said. “At all times, appropriate infection control precautions were taken around all of the persons quarantined at Miramar, including these three patients.”

Going forward, a CDC laboratory staff member will join quarantine field teams to make sure specimens are correctly labeled, Nordlund said.

On Monday, the patient returned to the hospital for treatment. The CDC said in a statement that it was investigating the person’s contacts and working “to assess if those contacts had high-risk exposures.”

Thirteen people have been sickened by coronavirus in the United States since the beginning of the outbreak.

The patient was one of hundreds flown back to the United States from Wuhan, China, the epicenter of a now-global outbreak that has taken more than 1,000 lives. While the virus’s toll remains concentrated in China, where officials report more than 42,000 cases, governments around the world have been scrambling to get their citizens out of the illness’s way and watch them closely for signs of infection upon return.

UC San Diego Health spokeswoman Yadira Galindo said in an email that the patient left the hospital “the same way they arrived with all precautions taken.” The person wore a mask and was escorted by federal officials who also had “protection,” she added.

“The patient did not interact with the environment after leaving the hospital room,” Galindo said.

Americans coming back from Wuhan are undergoing 14 days of quarantine meant to prevent them from spreading coronavirus in the community. The first group put under quarantine, at California’s March Air Reserve Base, was released Tuesday.

Health authorities have cautioned that a negative test result from the CDC’s laboratory does not necessarily mean someone will not get the virus.

“This is an accurate test,” Nancy Messonnier, who heads the coronavirus response at the CDC, said in a briefing last week. “A negative test most likely means a person is not infected. However, it may mean that an infection has not developed enough to be detected by the test.”

Messonnier told reporters that the testing is generally not meant to drive “individual decision-making” but rather to help public health officials understand the virus. If someone starts showing symptoms, though, she said, the testing may become more important on an individual basis.

Lena H. Sun contributed to this report.

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