ATLANTA (CNN) – If you think a negative test result means you don’t have a coronavirus, you could be wrong.
It may take days before a new infection appears in a COVID-19 test.
“We know that the incubation period for COVID-19 is up to 14 days. And before that, you can take a negative test and have no symptoms,” said emergency physician Dr. Leana Wen.
“But you can actually hide the virus and be able to pass it on to others.”
So if you want to get tested before you see friends or family, here’s what you need to know:
If I got infected yesterday, would I take a test today?
Probably not. A study in the medical journal Annals of Internal Medicine examined false-negative test results in people who actually had COVID-1
The study estimated that during four days of infection before symptoms usually begin, the probability of getting an incorrect / negative result from the Day 1 test is 100%.
On the day people began to show symptoms, the average false-negative percentage dropped to 38%, according to the study. Three days after the onset of symptoms, the false negative percentage dropped to 20%.
“The virus simply takes time to multiply in the body to detectable levels,” said Justin Lesler, senior author of the study and an associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.
“You can only be infected with a few virus particles, but they won’t be detectable until they can replicate to adequate levels to be detected,” he told CNN by email.
So, how many days does a person have to wait after a possible exposure to be tested?
“There is no strict rule, but the evidence suggests that testing before the third day after exposure is not very helpful,” Lesler said.
Can I be infected while testing negative?
Absolutely. “People feel like if you take a (negative) test, you come out of the woods. And somehow you don’t,” said Dr. Rochelle Valensky, head of the infectious diseases department at Massachusetts Hospital.
For people with COVID-19, symptoms can take up to two weeks, but the average time is about five days, Valenski said.
“You are usually thought to be most infected the two days before that day and the two days after,” she said.
One of the reasons this virus spreads so easily is that people can be infected without any symptoms. The US Centers for Disease Control and Prevention estimates that 40% of infections are asymptomatic, and 50% of transmissions occur before symptoms begin.
“It was one of the biggest Achilles heels of it. And it was one of the biggest challenges and unexpected things, because (with) my cousins it’s not true,” Valenski said.
“The reason we were able to control the SARS outbreak so quickly – although there were obviously many deaths there – but it didn’t turn into a pandemic, is that people don’t shed (the virus) until they get symptoms.”
Does the different tests for COVID-19 matter?
There are two main types of diagnostic tests that try to find out if you have an active coronavirus infection:
Molecular tests, such as PCR tests, look for the genetic material of the virus. Most of these tests are done with nasal or throat swabs, although some can be done with saliva, says the US Food and Drug Administration.
“This test is usually very accurate and usually does not need to be repeated,” says the FDA.
But the disadvantage of molecular testing is that the results can take some time – anywhere from the same day to a week after testing.
“For people who show symptoms, studies so far have shown that the accuracy of a molecular test to determine a positive case increases with each passing day,” said Pia MacDonald, an epidemiologist in infectious diseases at the nonprofit research institute RTI International.
But for infected people who do not get symptoms, the degree of accuracy is less clear, she said. “Studies on the effectiveness of molecular tests on asymptomatic people are very limited.”
Antigen tests they are often known as rapid tests (although some molecular tests are also fast). Antigen tests are not antibody tests that tell you if you are previously had the virus and have already developed antibodies against the infection.
Antigenic tests do not look for the genetic material of the virus as molecular tests do. Instead, they look for specific proteins on the surface of the virus.
The good news is that you can get antigen test results in less than an hour. The bad news is that you are more likely to get a false negative result with a quick antigen test.
“Positive results are usually very accurate, but negative results may need to be confirmed by a molecular test,” says the FDA.
“Antigenic tests are more likely to miss an active coronavirus infection than molecular tests.”
This may help explain some of the recent spread of COVID-19 related to the White House.
While staff members close to President Donald Trump and Vice President Mike Pence are often tested, White House officials often use rapid antigen tests, which typically have a higher incidence of false-negative results than molecular tests.
No matter what type of diagnostic test you use, you are usually more likely to get a false negative than a false positive.
“If the molecular test is positive, it’s an accurate reflection of the infected person,” MacDonald said.
“If it’s negative, it’s less reliable that the person is really negative. The same goes for antigen tests.”
Can I get tested at home?
Yes. Some home test kits are available, such as the Everlywell COVID-19 molecular test. Users take their own samples from a nasal swab and send them to a laboratory, which will send the results digitally within 24 to 48 hours of receiving the samples.
But taking any COVID-19 test too early can miss an infection, said Dr. Frank Ong, chief medical and research director at Everlywell.
“As testing capacity continues to increase, more and more asymptomatic or mildly symptomatic individuals have been tested, most of whom are likely to have lower viral loads in their clinical trials,” he said.
So what should I do if I want to see friends or relatives?
If you insist on seeing anyone who doesn’t live with you, it’s best to quarantine for 14 days in advance, Walenski said.
“If you do it right, you don’t need a test,” she said. “It’s probably the cleanest way to do it.”
To be clear: quarantining means staying home. This does not mean that you can execute orders.
“The grocery store and the quarantine do not belong in the same sentence,” Valenski said.
Lesler agreed that quarantine was best and any testing should be done intelligently.
“If you are visiting an adult family member and you have a reasonable risk of being exposed, there is no substitute for a 14-day quarantine,” Lesler said.
“I would at least wait 10 days (under quarantine) and I will have a negative test,” he said.
“If you are visiting a younger, healthy family member and have little chance of being exposed before or during a trip, then 5 or 7 days (quarantined) plus a negative test are likely to greatly reduce the risk, although there is no guarantee of safety . “
What should I do after a negative test?
It is important to strictly quarantine not only before your test for COVID-19, but after your test too.
“You should definitely stay in quarantine while you wait for the test results and make sure everyone you meet is on the same page for the infection risk control plan,” Lesler said.
There are already cases of coronavirus spreading in families just days after a person has been negatively tested, said Dr. Michael St. Louis, a member of the CDC community guidelines team.
He said everyone should remember to treat family from different households in the same way you treat unrelated friends or co-workers during this pandemic.
What is the safest way to celebrate Thanksgiving?
The best way to help everyone stay healthy is to celebrate the holidays remotely.
“I have three children. … And my parents will not join us this year,” Valenski said.
“It’s just awful. But what I really stick to is that my parents are pretty healthy and I would never forgive myself if I made them worse. And I’m just looking forward to 2021 when we can be together.”
CDC offers to celebrate with loved ones practically online. You can also make traditional Thanksgiving meals and deliver them “in a way that doesn’t involve contact” to relatives, neighbors, or those who may feel lonely.
Walenski said the small sacrifices made on this Thanksgiving will help ensure that everyone is healthy enough to sit at the table next year.
“At least they will be there next year, while irresponsible behavior now may mean they won’t be there later,” she said.
“Let’s do this so we can make a much better attempt to be together at the table, healthy, in 2021.”
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