DeWine’s false positive spotlights testing issues
Experts offer answers to common questions
Ohio Gov. Mike DeWine tested positive for COVID-19 Thursday using a rapid test – conducted as part of protocol to greet President Donald Trump at Cleveland’s Burke Lakefront Airport.
Two PCR tests taken later that day ultimately found DeWine tested negative.
“This is the same PCR test that has been used over 1.6 million times in Ohio by hospitals and labs all over the state,” DeWine said in a statement soon after the second test’s results were released.
DeWine’s COVID-19 scare underscores the fact that not all tests work the same way, nor do they always provide identical results. Even the same test taken twice can show contradictory outcomes.
“It’s not that a test is good or bad,” explained Dr. Gary Procop, the director of medical microbiology at the Cleveland Clinic. “Understand the test characteristics and use it correctly.”
Dr. Michael Mina, an infectious disease epidemiologist at the Harvard T.H. Chan School of Public Health, said DeWine’s ability to quickly get a second test showed “the system worked as it should.”
Every test sometimes gives false positives – which is better than telling someone they don’t have COVID-19 when they do – and being able to get a new result within a few hours meant he was inconvenienced only briefly. “It’s a whole lot better than if we weren’t testing him at all and he were positive.”
Here are answers to seven common questions about diagnostic COVID-19 testing. These tests are different than antibody tests, which are used to determine whether someone has had COVID-19 in the past, not an active case.
What is a rapid test?
A rapid test, explained Dr. Sheldon Campbell, a professor of laboratory medicine at Yale School of Medicine, is a catch-all term for any quick test. Any test that provides results in 30 minutes to one hour, he said, falls under the umbrella of rapid testing.
“Seriously, it’s not a specific techni
cal term,” he told USA TODAY.
In rapid testing, explained Procop, there are two kinds of tests – antigen tests and nucleic acid tests.
What is an antigen test?
An antigen test, Campbell said, looks for the proteins that make up the virus coating.
Think of the virus like an M&M, Campbell suggests. Antigen tests look for traces of the hard-shell exterior of the virus, so to speak.
While they provide results more rapidly, they are not a perfect test. The Food and Drug Administration states that antigen tests are more likely to miss active coronavirus infections, and thus, cannot definitively rule out whether someone has COVID-19. DeWine’s first test was an antigen test.
What is a PCR test?
A PCR (polymerase chain reaction) test searches for the viral genome. This test, explained Campbell, makes the virus easier to detect by “making a billion copies of a single target bit of the virus genome.”
To go back to the M&M analogy, Campbell likens the viral genome to the chocolate core of the candy.
These tests are more common and have a higher rate of accuracy.
Which test is more common, and which is preferable?
Here’s where things get a bit tricky. Generally, PCR tests tend to be more reliable.
“It’s both more likely to detect the SARS-CoV-2 virus and usually less likely to give a positive signal if no virus is present,” said Campbell.
But Procop says the type of test is less relevant than the context in which it is performed. In a screening context, a patient generally wouldn’t have symptoms of COVID-19, whereas in a diagnostic setting, patients get tested because they show symptoms.
“If you’re using a highly sensitive test in a diagnostic setting,” he said, “you don’t need follow-up. If you use it in a screening setting, you should have confirmation.”
Essentially, if you’re taking a test out of an abundance of caution – and don’t show any symptoms – it’s worth taking another one in the case of a false positive.
Why could you get false results?
The super-short answer, joked Campbell, is that “(expletive) always happens.” A number of factors contribute to the possibility that any lab test could result in incorrect outcomes. Each test provides its own slew of errors.
Antigen tests get false results because their procedure requires “sticking a labeled probe to the virus proteins.”
“Despite the best efforts of the people who design the tests, sometimes the probe sticks to non-virus sticky stuff in the sample,” Campbell said.
For PCR tests, the same sensitivity that contributes to its accuracy can create false positives. A bit of viral RNA from a previous patient can turn a test positive.
There are also human errors that come into play, whether lab specimens improperly collected or labeled, or manufacturing errors with testing kits.
Can you test positive after symptoms have gone away?
Absolutely. It remains unclear why that is, Campbell said, but a common theory he suggested is that “bits of noninfectious virus” slowly work their way out of the body even after symptoms have vanished.
“Some folks with COVID-19 stay positive for days or weeks after they get better,” Campbell said.
That’s also why the Centers for Disease Control and Prevention, explained Procop, changed its criteria for employees returning back to work, from two weeks to 10 days after a positive test.
“There’s a long tail of positivity after patients have recovered and we believe it’s clinically meaningless,” he said.
What can you expect when you get a test?
Expect the long swab. There are different nasal swabs with varying degrees of efficacy.
The long swab, otherwise known as the nasopharyngeal swab, is probably the best and is most common if you do demonstrate symptoms of COVID-19.
An alternative is the nasal mid-turbinate swab and a less-invasive “anterior nares” swab, which are less effective but sufficient.
Still, the long swab is the gold standard. Campbell puts it this way: “The swab goes farther up your nose than maybe you thought it should, and done right it burns when it’s up there. But it’s over quickly, and feeling the burn means you know your test was done right.”