That’s the early assessment of experts who have studied the rapid antigen tests that detect a protein on the surface of SARS-CoV-2. They caution that more information is needed, including how and why the tests were performed on Musk.

The controversy arose Friday, when Musk tweeted that he had been tested four times using Becton, Dickinson and Co.’s rapid antigen test, receiving two negative results and two positive findings. Musk, who traveled to Berlin last week on his private jet, said he had been experiencing cold-like symptoms.

“Something extremely bogus is going on,” Musk wrote. He went on to suggest without evidence in a subsequent tweet that such problems could account for the surge in virus cases that has been building in the U.S. since September.

Rapid tests have become more widely used in the fight against Covid-19, expanding access to testing because they provide results quickly and cheaply, and don’t need to be sent out to a laboratory. But the tests are also known to be less accurate than the more common, gold-standard polymerase chain reaction tests.

Musk’s contradictory test results could mean the Tesla leader is at the start or end of an infection, with a just barely detectable amount of virus in his system for the test to pick up, said Andy Pekosz, a professor of microbiology at Johns Hopkins University. Pekosz last month co-authored a study, paid for by the company, that reported Becton Dickinson’s test was very good at detecting infectious individuals, though the sample size was small.

If Musk is at the start or end of an infection, the scenario could be similar to what happened at the White House this fall, when President Donald Trump and other officials and guests fell ill with Covid-19 despite being screened with a rapid antigen test made by another manufacturer.

But it’s also possible that the assays used returned false positives for Musk, something known to occur with antigen tests. This can happen for a variety of reasons, including when they’re used in populations with low virus prevalence and when a test is improperly performed, as the U.S. Food and Drug Administration warned earlier this month.

‘Very concerning’

“That would be the scenario of most concern,” according to Pekosz. With false positives, “one would be of concern,” he said. “To have two is very concerning.”

Becton Dickinson said in a statement it was reaching out to learn more but that it stood behind its product, noting that the scientific community agrees no diagnostic test is perfect.

“There are many factors that could lead to a discordant result, including a low viral load,” the company said in its statement. “As we clearly state in our instructions for use, negative results should be considered in the context of a patient’s specific situation and confirmed with a molecular PCR assay if necessary for patient management.”

Musk said in a tweet that he is awaiting results of a PCR test.

There are also unanswered questions about why Musk received four of the tests in one day. Because the rapid tests are still in relatively short supply, it’s unlikely that someone who is not a celebrity businessman would have received that kind of treatment. Pekosz said he imagined Musk received a positive result initially and then had other tests done to try and confirm the result.

Correctly used

It’s also unclear whether the swab was taken correctly and the sequence of the tests, said Michael Mina, a Harvard epidemiologist who has advocated for wide use of antigen tests.

“Did he get two positives and then ‘use up’ the virus in his nose but continue swabbing?” Mina said in an email. “Point is, he didn’t give enough information to know if the test results are real or how to interpret it.”

There are major differences between the performance of different antigen tests, Mina said, which is why he’s been calling on the FDA to provide ratings for them and describe the best way each should be used.

Even the best-performing antigen tests will return false positives, according to Mina. One way to combat that, he said, is to sell antigen tests in a pack that also includes a few highly reliable rapid tests from another company, which can be used to double-check results.