LOS ANGELES (CBSLA) — After first testing positive for the novel coronavirus in March and spending 21 days in isolation, a Colorado nurse practitioner finally tested negative for COVID-19.

But seven days later, Lisa Merck was retested and, to her surprise, the result came back positive.

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“I feel like the test that came aback was a false negative,” she said.

And Dr. Adrian Cotton, chief of medical operations at Loma Linda University, said viral polymerase chain reaction tests — like nose and throat swabs — are not always accurate.

“You’re more likely to get a false negative than a false positive,” he said.

In fact, depending on the test, the reliability of the results can range from 60-90%.

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Cotton also said that nobody really knows if people can be immune to COVID-19.

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“If they have an infection to that virus, the body recognizes the virus, creates antibodies to it, so the next time the virus comes, it knows how to attack it and destroy it,” Cotton said. “We think that same thing would occur with this virus.”

Over the weekend, the World Health Organization said there was “no evidence” that people who have recovered from COVID-19 cannot get the virus again — though the organization later clarified its statements.

But South Korea and China have both shown a growing number of recovered patients once again testing positive.

“The question is whether it really is reactivation or low-level infection that was not detected for a period of time,” Dr. Stanley Perlman said.

Perlman has studied infectious diseases for more than 40 years and believes that, just like MERS, a person with more severe symptoms could have a stronger immunity to the virus if it attacks again.

“We think that you would be protected at least for some amount of time, especially if you had pneumonia,” he said. “If you had only the upper airway infection, you may get infected again.”

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Doctors have said just because a person has antibodies does not mean that they are immune to COVID-19, which is why the World Health Organization is pushing back on the idea of immunity cards — which Mayor Eric Garcetti has suggested as part of a reopening strategy.