Professional U.S. golfer Cameron Champ immediately withdrew from the PGA Tour's Travelers Championship last Tuesday and announced plans to self-quarantine for 14 days after testing positive for the coronavirus.
Five days later, he said he tested negative — three times — raising questions about the accuracy of the tests that are being used by professional sports to screen players before big ticket events and games.
After months of shutdown, the PGA resumed its newly revised schedule in June. As an outdoor, non-contact sport, professional golf is serving as an early testing ground for other sports leagues that may face a more difficult time resuming games in the middle of a highly contagious pandemic that primarily spreads through human contact.
Five PGA Tour golfers tested positive for the coronavirus since June and eight withdrew from the Travelers tournament, which ran under stringent new safety protocols and without on-site spectators from June 25 through June 28. Despite the precautions, several caddies have also tested positive.
"I feel great physically and I was obviously surprised and disappointed to learn of the test result," Champ said in a statement when he withdrew from the tournament. He's since said over Twitter than he doesn't have any symptoms.
As the number of new confirmed coronavirus cases continues to jump in several states across the U.S., some people, including Tesla CEO Elon Musk, are questioning whether inaccurate tests are artificially inflating state case counts.
"There are almost no false-positive tests with the RNA test," UC Berkeley infectious disease specialist John Swartzberg told McClatchy-Tribune news service. "At best he's confused, at worst he's prevaricating," Swartzberg said of Musk.
PGA Tour spokeswoman Laura Neal said Champ's test wasn't a false positive, something infectious disease specialists is rare.
"An initial positive is a positive test, and there is a medical explanation of what happens from there," Neal said in a statement, citing data from PGA's official coronavirus testing partner Sanford Health.
The PGA Tour uses a nasopharyngeal polymerase chain reaction (PCR) tests, which is a molecular diagnostic testing technique that detects the genetic material from the virus and is considered the gold standard in Covid-19 testing, she said.
She said a negative test result after an initial positive test could mean one of three things: the person is on the "upswing" of getting sick and has a low viral load, the person is simply a low-load carrier who never develops symptoms or the person is on the "downswing" meaning they're already recovering from the virus.
"Basically, given the accuracy percentage of the testing process we're using, we're confident the person has the virus in them and we want to be on the safest side possible," she said.
Molecular tests such as the nasopharyngeal PCR test used by the PGA Tour have the lowest false positive rates out of the three main types of diagnostic Covid-19 tests: molecular, antigen and serology, physicians say.
"To make a diagnosis of an acute coronavirus disease, you usually use the molecular test," said Dr. Gary Procop, a member of the American Society for Clinical Pathology board of directors and vice-chair of Pathology and Laboratories at Cleveland Clinic. He said antigen tests can be used to test for an active infection, but they aren't as accurate, and antibody tests only check whether you've previously been infected.
"No test is 100% sensitive. It's very rare to have a test that's 100% so we shoot for tests that are in the high 90's. 95% and up are good tests," he added.
Procop explained that there are two reasons for why a false positive case can occur. The first is when there are any cross-reacting substances. If a person is infected with another type of coronavirus, their antibodies can make them look like they have positive Covid-19 antibodies, resulting in a false positive test result.
The second reason is when a person has a very low viral load. When someone is infected with a disease, the virus can still be detected in their specimen for many weeks even after they recover and are no longer contagious.
"When you have very low levels of the virus or viral fragments, one day you might be positive. The next day, you might be negative. There's really nothing wrong with the test. It's just the function of the test," Procop said.
The lower the viral load, the less accurate the test, he said. "That's a coin toss between testing positive and negative. The test is fine. There's just very low levels of target."
Procop also said that a false positive test result occurs less frequently than a false negative. False negative results can occur from inadequate nasal swab sampling, poor specimen or testing someone too early in the disease. That's a lot more common than a false positive test, he said.
While false positive and false negative cases can happen, they are too rare to distort statewide testing data, according to Procop.
"What people have shown is when you have hospitalization outpace your testing, you know it's true disease," he said.
The molecular tests are "state of the art," Procop said. "The tests that we have are really good tests. I think this is as good as it gets."