Tests being used by US officials for suspected cases of coronavirus are not 100% reliable, officials said Friday.

One expert said: “If a person has HIV and I try their blood, I can tell you 100% whether they have HIV. That’s not even near the case with this.”

The US dramatically stepped up prevention measures Friday, barring foreigners who have recently traveled to China from entering the country.

US citizens will be allowed back, but will have to be quarantined for up to 14 days.

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Tests being used on US patients suspected of carrying the coronavirus ravaging China don’t always work, and throw up false results, according to the head of the Centers for Disease Control and Prevention (CDC).

Robert Redfield, the CDC’s director, pointed out problems with the test at a briefing on the virus, which originated in Wuhan, China, where some 200 people have died from it.

Speaking Friday at the White House, he said: “We’ve seen people who had a detectable virus, then they didn’t have a detectable virus, and then three days later they had a detectable virus.”

The problem further complicates the CDC’s job of isolating and controlling the virus, which has so far been confirmed in seven separate people in the United States.

Foto: Medics in Hong Kong help a woman and child during the coronavirus outbreak.sourceMiguel Candela/SOPA Images/LightRocket via Getty Images

In at least one case, a patient who had not been to China caught the infection, confirming that it is capable of spreading on its own within the US.

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At the same briefing, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, also expressed concern about the tests, which he said are worse than those for other diseases.

“We don’t know the accuracy of this test. People who came in were negative, then all of a sudden they were positive,” he said.

“If a person has HIV and I try their blood, I can tell you 100% whether they have HIV. That’s not even near the case with this.”

As reported previously by Business Insider’s Hilary Brueck, the coronavirus is difficult to diagnose.

Its symptoms can look a lot like the flu or a common cold, and common tests like checking somebody’s temperature are not definitive.

Instead, doctors need to tests samples of mucus and saliva in a lab. This can then be analyzed to see if it contains genetic material that matches the virus.

The CDC says it currently has no way to test for the virus before somebody starts displaying symptoms, meaning that there is no way to identify people during the virus’ incubation period, which is up to 14 days.

On Friday, the US government moved to bar non-US citizens who have been to China in the past 14 days from entering the US, in the hope of containing the spread of the disease.

US citizens will be allowed back into the country, but quarantined for as long as 14 days.

Aria Bendix contributed reporting for this article.