When Should You Get a COVID-19 Test? What About an Antibody Test?
By Kristen Fischer
As more testing for COVID-19 rolls out, you may be wondering whether you should get tested.
Tests for COVID-19 include the polymerase chain reaction (PCR) diagnostic test, which is a nasal swab. There's also the antibody test, a blood test that can tell whether you had an infection in the past.
So, should you wait till you have symptoms to go ahead and get tested? Or is it worth it to see whether you had an infection in the past?
Getting Tested for COVID-19
People exposed to the virus who have had close contact with a confirmed case should get tested whether or not they have symptoms, Amira Roess, PhD, a professor of global health and epidemiology at George Mason University, told Healthline.
"By identifying individuals who are positive early in disease progression before they develop symptoms and implementing public health interventions, we can prevent a large percentage of infections. This is key, because we have learned that asymptomatic infection is a key driver of this epidemic," she said. "Finding asymptomatic individuals will allow us to prevent them from spreading the virus."
On the flip side, people with no exposure history and no symptoms should not get tested, Roess adds.
The virus's incubation period is around 5 to 7 days, but it can be up to 14 days.
If you get tested too early after exposure, it can be possible that you have a false-negative test, notes Dr. Abraar Karan, an internal medicine doctor at the Brigham and Women's Hospital and Harvard Medical School.
A positive polymerase chain reaction (PCR) means that you have detectable viral RNA. "This does not necessarily say if you are infectious," Karan said.
Despite a positive PCR, studies have shown for some people there was little to no culturable virus after about 9 to 10 days. This means a person had an infection with the virus but tested positive long after they were no longer infectious to others.
However, PCR can remain positive for several weeks after active infection. "This means you are detecting viral RNA but not that you are infectious to others," Karan said.
The Centers for Disease Control and Prevention (CDC) says a positive test means you should isolate at home for 10 days.
Dr. Sophia Yohe, director of the University of Minnesota Molecular Diagnostics Laboratory and medical director for the Fairview/M Health COVID-19 testing lab, points out that the test only helps if you're going to be active in protecting yourself and others.
If you don't plan to isolate yourself if you're positive, the test is moot, says Yohe.
Antibody tests looking for immunoglobulin M (IgM) and immunoglobulin G (IgG) levels can tell whether you recently had an infection or had one in the past.
"Antibody testing should be done in individuals who have COVID-19 symptoms but did not have immediate access to a COVID-19 PCR test," said Dr. Amy B. Karger, medical director of the University of Minnesota Health West Bank Laboratory.
Typically, it takes about a week or two to develop antibodies after symptoms start. That's why the antibody test isn't ideal for diagnostic purposes if you've had symptoms for less than a week, she says.
Antibody testing isn't recommended for those within 8 days of symptom onset, Karger says.
"In these individuals, only the COVID-19 test (PCR test) should be used for acute diagnosis," she told Healthline.
People within that time frame may not have developed antibodies yet, and therefore there's a high risk for a false-negative result.
Additionally, there are issues with how accurate these tests are.
In early February, the Food and Drug Administration (FDA) authorized the emergency use of tests that can detect the new coronavirus.
This means the tests can be released before they're evaluated by the FDA for efficiency.
When to Get Both Tests
Federal guidelines recommend testing people with both tests if they present 9 to 14 days after symptoms start.
Antibody testing is useful for people with asymptomatic infections who experienced a COVID-19-like illness 14 days prior.
It's also useful if you were exposed to a person with COVID-19 14 days prior. In those cases, it can determine whether you had the virus in the past.
Experts haven't yet said that having the antibodies means you've got protective immunity, but evidence does support that, Karger says.
Additionally, it's possible to get a false positive for antibodies from the test, especially if you had no symptoms of COVID-19.
"Until we have additional data on these key factors, experts do not recommend assuming immunity with a positive antibody test," Karger said.
Physical distancing and mask use should still continue if you have a positive antibody test.
If you do test positive for antibodies, there's not much to do. The test simply provides information about exposure history to the new coronavirus and doesn't require any specific action, Karger says.
If you already got a PCR test and then go on to be exposed or develop symptoms, you should be retested.
The CDC advises healthcare workers who had a positive PCR test can return to work safely if two consecutive specimens collected within 24 hours apart come back negative.
Unfortunately, data is lacking on whether asymptomatic, non-exposed individuals who previously had a negative PCR test should get periodic testing, Yohe says.
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