Health and Wellness

What Covid-19 antibody and viral testing can and can't tell you about your health

A doctor in a protective suit taking a nasal swab from a person to test for possible coronavirus infection

With cheap at-home Covid-19 antigen tests in the works, a number of antibody tests quickly gaining approval for emergency use and Americans thinking about going back to work, you might be wondering if it's time to get tested? The answer is complicated.

Testing, both for the Covid-19 virus and its antibodies, doesn't replace the need for other Covid-19 prevention measures, such as social distancing or hand hygiene. However, at a time when people are craving more information about their health, a test result provide some people peace of mind.

Here's what experts want you to know about testing:

The difference between viral and antibody testing

There are two main categories of Covid-19 tests everyone is talking about. The first is a diagnostic test to check for an active infection: it checks for the presence of SARS-Cov-2, the virus that causes Covid-19, in your respiratory system.

During a Covid-19 diagnostic test, a clinician will insert a long swab into each side of your nose or mouth to retrieve a sample. (This video demonstrates what a nasopharyngeal swab looks like.) The test itself is very sensitive: it takes a matter of seconds, and results can be turned around in 24 hours, Beuther says. 

Even if you're not exhibiting symptoms, you could be shedding pieces of the virus. If the test comes back positive, then it is assumed that "you're sick and infected," Dr. David Beuther, a pulmonologist at National Jewish Health, tells CNBC Make It.

An antibody test, on the other hand, looks for levels of Covid-19 antibodies in your blood. When a person gets an infection, their immune system can produce specific proteins, called antibodies, to target the invader, in this case a virus. The antibodies that are associated with Covid-19 take one to three weeks to develop.

An antibody test isn't a way to diagnose an active Covid-19 infection. "We're really looking at whether you've already been sick and whether your body has mounted an immune response to the virus," Beuther says.

What it means if you tested positive for antibodies or negative for the virus

A positive antibody test result might seem like it gives you some invincibility, or at least allows you to relax a little, but that's not the case. All an antibody test tells you is whether you've been exposed to the virus in the past.  

Experts say it's too soon to know what level of antibodies could provide immunity from future infection, or even how long that potential protection would last. While the presence of antibodies likely means that you had Covid-19 at some time, "we're reluctant to say that you're immune and you won't get infected again," Beuther says.

"It's okay to get tested for antibodies, but take it with not just a pinch of salt, but a fist of salt," Omer says. "There's no harm, as long as you interpret [the results] very sufficiently, and you don't relax the other restrictions."

Additionally, with antibody testing, the chance of getting a false-positive test result is high. In an area where very few people have had Covid-19, for example, a higher percentage of positive antibody results may be false-positives, according to the CDC.

"In a population where the prevalence is 5%, a test with 90% sensitivity and 95% specificity will yield a positive predictive value of 49%," according to the CDC. (Sensitivity refers to a test's ability to correctly generate a positive test, while specificity refers to a test's ability to correctly generate a negative result.) "In other words, less than half of those testing positive will truly have antibodies." 

"A positive test result is more likely a false-positive result than a true positive result," according to the Infectious Disease Society of America.

But that doesn't mean antibody testing is useless. In some circumstances, healthy people who have antibodies in their blood and have tested positive for Covid-19 virus in the past (but are now symptom-free) can donate blood plasma, which could be used as a treatment for Covid-19.

As for a negative viral test, a recent study from Johns Hopkins University suggests that these tests have a 20% false-negative rate, because the timing of the test can affect results. That means it's possible to have the infection and test negative.

"False-negative results are far more consequential [than false-positives], because infected persons who might be asymptomatic may not be isolated and can infect others," Steven Woloshin, MD, MS, a professor of medicine and community and family medicine at Dartmouth's Geisel School of Medicine, said in a release

Who should get tested

If you have symptoms of Covid-19 (a fever, cough or shortness of breath, for example), or if you've had close contact with someone who was infected with the virus, the Centers for Disease Control suggests you should get tested for the Covid-19 virus.

An antibody test is conducted after someone has recovered fully from Covid-19. "Antibody testing is really useful," especially when it's done on a group, like an office or city healthy department, because it can show how many people in a community got infected, Dr. Saad Omer, director of Yale Institute for Global Health and professor of medicine in infectious diseases at Yale School of Medicine, tells CNBC Make It.

What protesters need to know about testing

New York Gov. Andrew Cuomo urged anyone who had been to a protest to get tested for Covid-19 in a press conference June 5. He also opened 15 testing sites in New York City specifically for protestors.

At a large protest, it can be difficult to maintain social distance in a crowd, plus you spread more respiratory droplets while shouting or chanting. Another risk? If someone at a protest got sick, it would be challenging to perform contact tracing to determine how many people they encountered, Beuther says. If you're going to protest, "the key is to do what you can to wear a mask and protect yourself with hand hygiene," he says.

In the immediate aftermath of a protest, you might be curious to know if you've been infected, especially if people weren't keeping their distance or wearing appropriate face coverings. It can take a few days for the virus' DNA to be detectable, so you should wait at least three days after a protest to get tested, Omer says. Even if you have mild symptoms after exposure, it's helpful to get an antibody test, he adds. 

If you don't develop symptoms following a protest, Beuther says that you don't necessarily need to get tested. However, if you're someone who has elderly or high-risk people in your home, or you have a job that exposes you to lots of people, then it makes sense to get tested, he says.  

What to know if you get tested

Remember: A test result can only provide a snapshot about your status at one moment in time.

"With all of these tests, we have to think about that the Covid-19 infection is a dynamic process," meaning it's active and can change, Beuther says.

For example, if someone got tested too early in the incubation period, it's possible that they could get a false-negative Covid-19 diagnostic test. Similarly, if someone takes an antibody test while still infected with the virus, the test may not pick up the antibodies.

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