From swabs to antibodies: How to understand your coronavirus test results

Americans are being swabbed by the thousands to learn if they have covid-19, the disease caused by the novel coronavirus. It's how many are determining their risk of contracting or spreading the virus to someone else.

Experts say testing is a vital component to controlling the outbreak, but one test result still isn't a green light to visit vulnerable friends or family members. The nature of covid-19, the time it takes for someone to develop symptoms and the varied ways the virus affects people make each test a snapshot in time more than a definitive answer.

"I personally wouldn't consider a single test a license to go see my parents, who are older and would be at higher risk," said Carl Bergstrom, a University of Washington biology professor who studies infectious diseases.

The viral swab tests, seen at drive-through clinics across the country, tell people whether they're infected with the novel coronavirus on that particular day, said Lucy Wilson, an infectious-disease specialist and a professor at the University of Maryland Baltimore County.

It takes time for the coronavirus to replicate to a critical mass for a swabbing test to detect it. The time this process takes varies from person to person and ranges from two to 14 days, experts say.

"The problem is this virus is a strange virus," Bergstrom said. "The timing varies a lot in people."

Imagine that you become infected today and are tested tomorrow; Bergstrom said "we have every reason to believe you're going to test negative, even though you're infected."

On average, people tend to show symptoms or test positive for the virus about five days after exposure, Wilson said.

"It's just like a pregnancy test," Wilson said. A pregnancy test can detect only certain hormones after a certain number of days or weeks, but it doesn't mean you're not pregnant.

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If you do start developing symptoms, such as a fever, sore throat or loss of smell, experts say you should absolutely get tested for the coronavirus. Wilson said people need to determine whether the symptoms they're experiencing are a result of the coronavirus or are another illness - such as strep or the flu.

The Centers for Disease Control and Prevention advises anyone with symptoms to stay home and self-isolate as much as possible.

If you've been in contact with someone who has covid-19 but you don't have any symptoms, Wilson said, you should consider a 14-day self-quarantine and discuss whether to get tested with your health-care provider.

» FAQ: Your coronavirus questions, answered.

As the Atlantic reported last month, it's still not clear how accurate viral tests are for people who haven't developed symptoms. There's a degree of uncertainty, even with a negative test result, and not a lot of data to determine exactly how early a swab test can start to detect the infection for a person showing no symptoms.

On top of all this, the rising demand for more testing has led to week-long delays for results. An average of 685,000 people were tested per day last week, according to data collected by the Covid Tracking Project and reported by the New York Times. The White House aims to reach 1 million tests a day by the fall.

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Experts say the backlog in some parts of the country makes the results useless for efforts to control the spread of the virus. Generally, people who have the virus are symptomatic for around six days, Bergstrom said. If the results take five days to come back, there's only so much a person can do to protect those around them. Bergstrom added the results become "absolutely useless" for efforts to quarantine or to trace contacts.

This is why, regardless of testing, public health experts continue to stress wearing masks in public and physical distancing. In the absence of test results, or symptoms, keeping your distance from others helps in mitigating the spread of the disease.

There are still not enough tests for everyone to be regularly screened for the virus, said Erica Sohs, an infectious-disease expert and professor at the University of Nebraska Medical Center. Researchers at Harvard's Global Health Institute say the United States needs to triple, if not quadruple, the testing capacity to contain the virus. The actual number recommended by the institute varies day-to-day because the methodology is dependent on a number of variables.

One potential solution as grade schools and universities weigh reopening in the fall is pool testing, where swabs from a group of people are tested all at once to save time and conserve supplies.

"If you wanted to do surveillance testing just to make sure there's not a silent outbreak going on in a school, having pool testing would be helpful," Sohs said. "I wish we were talking more about that."

People who think they already had covid-19 in the spring but did not go to a hospital are getting antibody tests, also known as serology tests. Bergstrom said some just want to know whether that bad cold they had a few months ago was actually the novel coronavirus.

Antibody tests can tell whether someone has already been infected with covid-19 by using a blood sample to identify the proteins a body produces one to three weeks after an infection, according to the CDC.

"Right now, we don't really know what a positive antibody test means in terms of the degree to which you're protected."

In general, antibodies help immune systems fight off any future infection from the same virus, but it's not clear how much protection covid-19 antibodies can provide or how long the protection might last. That's because immunity varies depending on the pathogen. Antibodies from a measles infection will provide a person lifelong immunity. Meanwhile, antibodies for a strain of influenza won't protect the body for nearly as long.

Antibody tests can also provide a false positive reading, meaning the test indicates you have antibodies from covid-19 when that's not the case. The false positive may just mean your body has antibodies for another coronavirus, like one that causes the common cold.

"We're just not there yet with the accuracy of the antibody test," Wilson said.

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At the moment, experts can't say if antibodies from a past covid-19 infection provide someone immunity or even temporary protection from the virus. The virus is still so new. Because of this, Bergstrom said positive antibody tests shouldn't be used as "a license" to return to the office or other group activities.

"Right now, we don't really know what a positive antibody test means in terms of the degree to which you're protected," Bergstrom said.

What antibody tests can provide is a broader understanding of the progression of an outbreak. In the District of Columbia, fewer than 6 percent of residents have tested positive for antibodies from the coronavirus out of 13,706 blood samples. The results show public health experts who has and hasn't been exposed to the virus.

According to the FDA, people with antibodies from the virus may be able to donate their plasma to be used as "a possible treatment" for those with the disease.

For anyone still waiting for their test results, experts say it's important to be aware of the caveats. The tests can determine only so much. Beyond what we know, Bergstrom said, everyone must weigh the risks and mitigate their own possibility for exposure.

"We can't all stop living our lives entirely," Bergstrom said. "We have to make decisions about the risk we want to take on."

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The Washington Post’s Steven Johnson contributed to this report.