There’s a lot of talk about tests for COVID-19, as well as a lot of confusion about what the results might mean. If someone tests negative for COVID-19, does that mean they aren’t sick? If someone tests positive for antibodies against COVID-19, does that mean they are immune?
Right now there are two main types of COVID-19 tests: diagnostic and serological. A diagnostic test works by looking for the virus’s genetic material, which would be found during an active infection. A serological test works by looking for antibodies against the virus, which are only to be found later in the infection, as well as after you’ve recovered.
“Each test has strengths and weaknesses,” said Pedro Piedra, MD, a professor of molecular virology, microbiology and pediatrics at Baylor College of Medicine.
Diagnostic tests look for the virus
If you’ve heard about the test where they stick a swab in your nose and reach it all the way to the back, that’s a diagnostic test, which works by looking for the virus itself. There are a number of diagnostic tests that have been developed, all of which work by looking for genetic material from the virus.
A positive result means the virus was detected in the patient, which indicates an active infection. Viral particles are found in a patient when they are showing symptoms, as well as for a brief period before and after, during what is known as viral shedding.
But if you’ve been tested for COVID-19, and got a negative result, that does not necessarily mean you aren’t infected.
“The major issue is often poor quality samples,” Piedra said. Getting enough of a sample is tough, especially under less-than-ideal circumstances. He also advises talking with your doctor about the test results, and what these results mean.
Serology tests look for antibodies against the virus
Serological testing has been getting a lot of attention lately, mostly because it’s being talked about as a measure for re-opening society, by identifying people who are immune to COVID-19. Serological tests work by looking for antibodies against the virus that causes COVID-19. Those antibodies are an indication that a person was exposed in the past, whether they remember getting sick or not.
There are two types of serological tests: one which looks at IgG antibodies and one which looks at IgM antibodies. IgM antibodies are your immune system’s first line of defence and are produced earlier in the infection. IgG antibodies take a while to build up and can be found in your body after the infection is over.
A negative serological test doesn’t necessarily mean you aren’t infected.
“It takes a while to build both types of antibodies, so in an acute case, you wouldn’t detect them, and have a false sense of security,” says Luis Ostrosky, MD, an infectious disease specialist at McGovern Medical School at UTHealth in Houston.
Serology tests a possible indication of immunity
The big conversation surrounding serological testing is the idea that a person who tests positive is immune to COVID-19. In theory, this would mean that a healthcare worker could safely care for COVID-19 patients, essential workers could be cleared to go back to work, and others might be able to come out of quarantine or isolation.
As hopeful as this idea sounds, it may not be that simple. “There’s a lot we don’t know yet,” Piedra says. Some of the unknowns include whether a person can get re-infected with COVID-19 as well as how long this immunity lasts.
If you do test positive for antibodies, don’t think this means you can go back to your normal life. “You should still have a conversation with your healthcare provider,” Ostrosky says.
Why it is so important to confirm COVID-19 infection
Testing, of both varieties, is still in short supply globally, which means there are people with COVID-19-like symptoms who haven’t been able to get an official diagnosis. Even in these situations, Ostrosky and Piedra are both adamant it’s important to confirm it was indeed COVID-19 before making any assumptions. If you have been sick with COVID-19 symptoms, but haven’t had an official diagnosis, that doesn’t mean you are now immune.
“It is very important to know for sure if you have had it or not, as it can give you a false sense of security and put you at risk for getting it,” Ostrosky says, while Piedra notes that there are many respiratory viruses that mimic mild or moderate symptoms of COVID-19.
If you have been sick with COVID-19 symptoms, but haven’t had an official diagnosis, that doesn’t mean you are now immune, and that doesn’t mean you can stop measures such as hand-washing and physical distancing.
“Do not let your guard down simply because you think you have been infected,” Piedra says.
Until then, we all have to do our part to flatten the curve, to help protect the safety of healthcare workers, as well as the health of our community. So wash your hands, practice physical distancing, and stay at home.
“We are in this for the long haul,” Piedra says.