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Home / Health / Covid-19 Test: This is why you may be infected and still give negative results

Covid-19 Test: This is why you may be infected and still give negative results



It may take several days before infection appears on the Covid-19 test.

“We know that Covid-19’s incubation period is up to 14 days. And before that, you can test negative and have no symptoms,” emergency doctor Leana Wen told CNN. “But you may actually be carrying the virus and possibly pass it on to someone else.”

So if you want to check for prevention before meeting friends or family, here’s what you need to know:

Sure is not. A study in the medical journal Annals of Internal Medicine examined false negative test results of people who actually had Covid-19 disease.
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Research estimates that in four days of infection before Symptoms usually begin, the probability of getting an incorrect / negative test result on Day 1 is 100%.

By the day people started showing symptoms, the average false negative rate dropped to 38%, according to the study. Three days after symptoms began, the false negative rate dropped to 20%.

“Viruses only need time to multiply in the body to detectable,” said Justin Lessler, senior author of the study and an associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

“You can be infected by just a few viral particles, but they won’t be detectable until they have had time to replicate to the right level to be detected,” he told CNN via email.

So, how many days should a person wait after being exposed for the test?

“There’s no hard and fast rule, but evidence shows that checking before the third day after exposure doesn’t help much,” says Lessler.

Can I get infected when I test negative?

Sure. “People feel as if if you test (negative) you are in the woods,” said Dr Rochelle Walensky, head of infectious diseases at Massachusetts General Hospital. And you don’t.

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For people who are sick with Covid-19, symptoms can take up to two weeks to appear, but the average time is about five days, Walensky said.

“People often think you’re most contagious two days before that date and two days after,” she says.

One reason why this virus spreads so easily is because people can be infected without any symptoms. The US Centers for Disease Control and Prevention estimates that 40% of infections have no symptoms and 50% of infections occur before symptoms begin.

“It was one of the biggest signs of the Achilles heel, and it was one of the biggest challenges and undesirable things because (with) its cousins, this didn’t,” Walensky said. it’s correct”.

“The reason we have been able to control the outbreak of SARS so quickly – although there are obviously so many deaths there – it hasn’t turned into a pandemic is because people haven’t eliminated the (virus) until they have symptoms. ”

Are the different types of Covid-19 tests important?

When it comes to diagnostic tests – those that detect whether you have an active coronavirus infection – there are two main types:

Molecular test, such as a PCR test, looks for the virus’s genetic material. Most of these tests are done with nasal gauze or throat swabs, although some can be done with saliva, the US Food and Drug Administration says.
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“The test is usually highly accurate and usually doesn’t need to be repeated,” the FDA said.

But the downside of molecular testing is that the results can take a while – anywhere from the same day to a week after the test.

“For people with symptoms so far studies have shown the accuracy of molecular testing to find out,” said Pia MacDonald, infectious disease epidemiologist at the RTI International Nonprofit Research Institute. Positive cases increased every day after exposure.

But for those infected with no symptoms, the exact rates are less obvious, she said. “Studies of the performance of molecular testing in asymptomatic people are very limited.”

Antigen test often referred to as a rapid test (although some molecular tests are also very rapid). The antigen test is not an antibody test that tells you if you have one before has a virus and has developed antibodies to fight the infection.

An antigen test does not look for the genetic material of the virus, like a molecular test. Instead, they look for specific proteins on the surface of the virus.

The good news is you can get the results of the antigen test in less than an hour. The bad news is that you are more likely to get a false negative with a rapid antigen test.

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“A positive result is usually highly accurate, but a negative result may need to be confirmed by molecular testing,” the FDA said.

“Antigen tests are more likely to miss an active coronavirus infection than molecular tests.”

This may help explain some of Covid-19’s recent spread with regard to the White House.

While employees close to President Donald Trump and Vice President Mike Pence are routinely tested, White House staffers typically use rapid antigen tests, which typically have higher rates of false negatives than testers. molecular solution.

No matter what type of diagnostic test you use, you are usually more likely to get a false negative than a false positive.

“If a molecular test is positive, it’s an accurate reflection of whether a person is infected,” MacDonald said. “If it’s negative, it’s less reliable that the person is actually negative. The same is true for antigen tests.”

Can I test myself at home?

It’s correct. There are a number of home test kits available, such as the Everlywell Covid-19 molecular test. Users sample a cotton swab for themselves and send it to a lab, which will send digital results within 24 to 48 hours of receiving the sample.
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But doing any kind of Covid-19 test too soon can miss an infection, says Dr. Frank Ong, medical and scientific director at Everlywell.

“As testing continues to increase, more and more people with no symptoms or mild symptoms have been tested, most of whom have lower viral loads in clinical samples,” he said. their”.

“My best advice is to treat yourself as if you were likely to be infected after suspected exposure. Quarantine, wear a mask to cover your nose and mouth, and practice remedies. If you want to take additional precautions or you believe you may have been exposed, get tested for viruses using the FDA-approved test for Covid-19. “

So what should we do if we want to meet friends and relatives?

If you insist on meeting your loved ones on Thanksgiving, quarantining yourself for 14 days is probably the safest way for you, Walensky said.

“14-day quarantine – with real isolation – if you do it the right way, you don’t need testing,” she said. “That’s probably the cleanest way to do it.”

To be clear: Isolation means being at home. It doesn’t mean you can run errands.

“Grocery” and “quarantine” don’t belong to the same sentence, Walensky said.

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She added testing to your list of precautions “is a helpful thing, but only if it’s done properly.” “And if you don’t know exactly when or how to use it, chances are you’re giving yourself a false sense of security.”

Lessler agrees that quarantine is best and testing should be done intelligently.

“If you are visiting an older family member and are at reasonable risk of exposure, there is no substitute for 14 days of isolation,” says Lessler.

“At least I would wait 10 days and test negative. If you visit a younger, healthy, and less likely to be exposed family member before or while traveling, then 5 or 7 days ( quarantine period) plus a test negative can reduce many risks, although it does not guarantee safety Either way, you should definitely stay on quarantine while you wait for the test results and make sure everyone you are meeting are on the same page about your infection risk management plan. “

Dr. Michael St. Louis, a member of the CDC Community Guidance Group, says there have been cases of corona virus spreading in families just days after a person tested negative.

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He said that people have to remember to treat families from different households the same way you treat unrelated friends or colleagues working in this pandemic.

Of course, the best way to help keep all of your loved ones safe is to celebrate it remotely.

“I have three kids. … And my parents won’t be joining us this year,” Walensky said.

“It’s terrible. But what I’m really trying for is that my parents are quite healthy, and I will never forgive myself if I harm them. And I’m just looking forward to a year 2021 when we are. can be together. ”

CDC suggests celebrating with loved ones on a virtual network. You can also make traditional Thanksgiving dishes and deliver them “in an unrelated way” to relatives, neighbors or people who might feel lonely.

Walensky said these small sacrifices during Thanksgiving will help ensure everyone will be healthy enough to sit at the table next year.

“At least they’ll be there next year, while irresponsible behavior now may mean they won’t be here later,” she said.

“Let’s do this so we can have a much better shot of being together, healthy, in 2021.”

CNN’s Lauren Mascarenhas contributed to this report.


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