By Robert Wall

Tesla Inc. Chief Executive Elon Musk, who has repeatedly played down the risk of the coronavirus since early in the pandemic, says he tested both positive and negative for Covid-19 on Thursday and raised questions about the validity of such testing more broadly.

Mr. Musk said on Twitter he was experiencing cold-like symptoms and, when taking four of the same tests administered on the same machine, had two results return positive and two negative.

"Something extremely bogus is going on," he said.

The four Covid-19 results Mr. Musk said he received came from rapid-response tests. They aren't considered as accurate as PCR, or polymerase chain reaction, tests that typically involve a nasal or throat swab.

Health officials in the U.S. are reporting higher infections, as well as more hospitalizations due to Covid-19 that rose to a record 67,096 Thursday, according to the Covid Tracking Project.

The symptoms Mr. Musk said he had experienced were mild "sniffles & cough & slight fever past few days." He later tweeted he wasn't feeling any symptoms after taking over-the-counter cold medicine.

If a diagnosis is confirmed, Mr. Musk would become one of several CEOs to reveal they contracted the virus. Morgan Stanley CEO James Gorman, NBCUniversal CEO Jeff Shell and Hewlett-Packard Enterprise Co. CEO Antonio Neri revealed positive diagnoses earlier this year.

Howard Willard, former CEO of Altria Group Inc., took a temporary medical leave after a positive diagnosis in March and announced his retirement in April after a rocky two-year tenure leading the Marlboro maker.

Through the pandemic, management teams have worked to fortify succession plans and review backup operating plans when critical employees fall ill.

In March, Mr. Musk said there would likely be close to zero new Covid-19 cases in the U.S. by the end of April. The outspoken CEO also said in March: "My guess is that the panic will cause more harm than the virus, if that hasn't happened already."

Mr. Musk disclosed the test results soon after a rocket being developed by Space Exploration Technologies Corp. that he also runs suffered a test anomaly in Texas. A different SpaceX rocket awaits a planned Sunday launch at the National Aeronautics and Space Administration's Kennedy Space Center in Florida to carry four astronauts to the International Space Station -- the launch was scheduled for Saturday, but got delayed as a result of onshore winds and for booster-recovery operations.

NASA Administrator Jim Bridenstine on Friday said it was the agency's policy that when someone tests positive, they self-isolate, and that it was looking to SpaceX to handle any appropriate contact tracing. "If there are adjustments that need to be made, we will make them," he said.

The astronauts scheduled to launch on the Falcon 9 rocket have been quarantining and, Mr. Bridenstine said, he wasn't aware of any contact between them and Mr. Musk. The crew, he said, "should be in good shape."

Mr. Musk on Twitter raised questions about the accuracy of the PCR test and the rates of Covid-19 false positives. He was met with a chorus of Twitter followers who also proclaimed skepticism about the tests and theories that Covid-19 infection rates were falsely elevated.

"The guy is spreading more misinformation on Covid than almost anyone else out there," said Ashish Jha, professor of health services and dean of the Brown University School of Public Health. "Things are horrible and he is actively using his platform to say things that will lead to more infections and deaths. It is deeply irresponsible and he should be better than this."

Mr. Musk and Tesla didn't respond to a request for comment.

On Thursday, Alameda County, near San Francisco -- where Tesla's lone U.S. car factory is based -- warned of rising coronavirus cases and potential new restrictions to combat the disease. County health officials earlier this year ordered Tesla to temporarily close the car plant.

On an earnings call in April, with the U.S. plant shut, Mr. Musk railed against local shelter-in-place restrictions aimed at slowing the spread of the virus.

"Give people back their goddamn freedom," he said.

When Tesla reopened the Fremont plant in May, it had put in place safety protocols that county officials inspected. Workers have reported instances of Covid-19 cases among workers at the facility, though neither Tesla nor local authorities have commented on the scale of infection among the vehicle maker's staff.

The response to the pandemic has become a polarized issue in America. Mr. Musk retweeted several comments on his disclosure that sought to raise questions about the accuracy of tests.

The test, which Mr. Musk said was from Becton, Dickinson & Co., is one of several authorized antigen tests, which search for virus proteins in patient samples and can deliver a result in about 15 minutes. The tests tend to be less precise than laboratory-based PCR tests. But they are good at identifying those that have higher viral loads and are likely most infectious, public-health experts say.

"We are aware of the tweet and are reaching out to learn more, consistent with our quality management process," a spokeswoman for Becton Dickinson said. "We stand by the quality, utility and science of our system and assay. There are many factors that could lead to a discordant result, including a low viral load."

False positives and false negatives can occur with any clinical test. False negative results, or failing to pick up a present infection, are more common with antigen tests, but false positives can also occur. Public-health authorities sometimes recommend a confirmatory PCR test and say that test results should be looked at in conjunction with other pieces of information, such as symptoms and potential exposure.

Becton Dickinson in September said it was investigating reports from nursing homes that federally provided rapid coronavirus testing equipment from the company was producing false-positive results in some cases.

Mr. Musk said he was taking a more thorough PCR test at a separate laboratory and was expecting to get results within about 24 hours.

Tesla shares were down about 2% Friday afternoon.

Despite the turmoil of recent months and the wider global economic slowdown from the pandemic, Tesla has been navigating the health crisis with little apparent impact. Vehicles deliveries in the second quarter fell compared with the year-ago period.

The pandemic at one point threatened to derail Mr. Musk's plan to boost Tesla deliveries by about 36% this year with the closure of the plant. Mr. Musk fought to reopen the factory, and Tesla last month said its goal of delivering more than 500,000 vehicles this year could still be attainable. The company is on track to post its first full-year profit in 2020 despite the pandemic, according to analyst estimates.

--Bowdeya Tweh and Brianna Abbott contributed to this article.

Write to Robert Wall at robert.wall@wsj.com

(END) Dow Jones Newswires

11-13-20 1717ET