Visby’s pcr test, covid-19 rapid tests: can you get corona twice?


Are antigen tests less accurate than other viral tests? It depends on your definition of “precise”. Here’s a breakdown.

WASHINGTON – As of December 7, 2020, the Food & Drug Administration has approved emergency use authorizations for nearly 300 different tests, including diagnostic and serology tests.

About two-thirds of these tests are traditional molecular tests, such as the PCR test, which can be run in a lab in a few hours but take a few days to get results.

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But only seven antigen tests, commonly referred to as “rapid tests”, have obtained emergency use authorization. These are tests that can take as little as 15 minutes and can be performed without a lab.

For months, the antigen tests sparked skepticism and public criticism for producing false results.

Elon Musk has publicly questioned the accuracy of antigen testing on Twitter in November.

“Something extremely wrong is going on,” he said. “Has been tested for covid four times today. Two tests came back negative, two came back positive. Same machine, same test, same nurse. Rapid BD antigen test.”

Musk’s tweet drew comments like these:

Search online and you’ll find dozens of similar claims that antigen testing is “known” for less reliable results. Thus, our verification team verified the claim.

Are antigen tests less accurate than other tests, like PCR?

Our Verify researchers asked this question of two infectious disease experts:

  • Dr Hanna Akselrod, Assistant Professor of Medicine and Co-Director of the COVID-19 Recovery Clinic at George Washington University
  • Dr Stuart Ray, Professor of Medicine at Johns Hopkins University.

We have discovered that this is a difficult question to answer without defining the term “correct”.

“When you ask me if the antigen tests are accurate, are they accurate for what purpose?” Ray said. “And I’ll give you some choices.”

Here is the list proposed by Ray:

  • Are they an accurate measure of whether someone is infectious with SARS-CoV-2?
  • Are they an accurate measure of whether someone has or recently had SARS-CoV-2?
  • Are they an accurate measure of whether or not a person’s current symptoms are due to a SARS-CoV-2 infection?
  • Are they a good predictor of whether that person will shed this virus tomorrow or the next day, because they were exposed two days ago?

So our researchers at Verify took a step back, starting from the beginning:

What information are PCR tests and antigen tests designed to give you?

According to the FDA, molecular tests, like PCR, check if the sample you collected contains the genetic material of SARS-CoV-2.

Antigen tests check for the presence of virus-specific proteins.

Both are designed to tell you if you have an active infection.

“Whether you do an antigen test or an RNA test, what you are testing is for the presence of the virus,” Ray said. “And the tests have different sensitivities, they have different abilities to detect small amounts because what we are detecting are molecules.”

In general, how do these tests work? The answer lies in statistics.

“When we take a close look at it in the world of math and immunology, there are actually two metrics that are really close to our hearts, ‘sensitivity’ and ‘specificity’,” Akselrod said.

“A sensitive test is one that will correctly detect the greatest proportion of cases of the disease that you are trying to find,” she continued.

“So if you’re trying to diagnose a disease, you don’t want to miss any cases. You want to identify and select the highest proportion of people with that disease to have a positive test result… the idea of ​​specificity is you want to also that this test accurately identifies who do not have the disease. And when you have a rare disease, or one that’s easily confused with other entities, you want to prioritize specificity, you want to test that won’t turn positive unless the person really has the disease you’re looking for. . “

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Basically the test sensitive enough to detect COVID and specific enough to determine that you actually have COVID and not another virus.

If any of these are turned off, you may end up with false positives or false negatives.

Molecular tests are very sensitive and very specific, which means you can be quite confident in the results.

Dr Akselrod calls them “the gold standard”.

Antigens are very specific, but generally less sensitive than molecular tests, according to the FDA.

The FDA sent a letter in November to lab workers and healthcare providers, fearing that antigen tests used in places like nursing homes could give false positives.

When antigen testing first came out in May 2020, the FDA also warned about false negatives.

As our audit team found out, even though the tests work very well and even though the tests are managed correctly, outside factors such as the population tested, the prevalence of the virus in the community, and how long a patient has had. contracted the virus, can contribute to false results.

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When should you get tested for the most accurate results?

The PCR test is really good at telling you if the virus is in your system. The antigen test, according to Ray, might be more reliable in telling you if you’re contagious.

Because antigen tests are less sensitive than PCR tests, antigen tests work best when you are at the height of your illness, but not as well at the beginning or at the end.

The antigen test should turn positive at the top of the curve when your viral load is highest, which is when you have a large amount of virus in you, but it may not turn positive until your viral load is over. symptoms do not start, or once you start to recover, and have less virus in you.

This is consistent with what our Verify researchers found in the clearance documents provided by the testing companies to the FDA and the CDC information.

“The sensitivity of antigen tests varies but is generally lower than that of most NAATs,” writes the CDC Online. “The level of antigen in samples taken either before symptoms appear or late in infection may be below the test virus detection limit. This can cause a negative antigen test result, while a more sensitive test, such as most NAATs, can return a positive result. “

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With a PCR test or an antigen test, if your test is negative, Ray says that doesn’t necessarily mean you don’t have the virus. It could mean that you have it, but just got tested too early or too late.

“None of the tests can really tell us if that person will be positive in a few days, because we don’t know where they are on those curves, the curve goes up and down, and if you don’t know where you are on that upward curve. and top-down, then we can’t say what the test means, ”Ray said.

Because antigen tests are generally less susceptible to the virus, the window for getting a true positive result is narrower, Ray said.

So … is one test more accurate than another?

“No, I think it depends on your goal,” Ray said. “If you ask, ‘Am I contagious today?’ the antigen test in this situation would be more accurate for infectivity. If your question is “Do I have the virus in my body?” the RNA test would be more accurate, but it wouldn’t tell you if you are contagious for d ‘other people.”

Even so, no test is 100% accurate.

“They just aren’t designed to work that way,” Akselrod said. “There are tests that are damn close to 100% accurate, the ones we use in hospitals and medical settings, molecular PCR tests, and we have the advantage of being able to test and re-test and validate against additional methods. “

Whether or not a result is a true positive or a true negative comes down to the performance of that individual test, how the test was administered and managed by health workers and laboratory staff, the timeline of your test. infection and the population tested.

There are advantages and disadvantages to both diagnostic tests. Although PCR tests are considered the “gold standard”, they can take longer and often need to be performed in a laboratory.

Antigen tests are generally less sensitive but are cheaper to perform, do not require a laboratory, and can be performed in minutes. But if your test is negative, your doctor may order a molecular test to confirm the results.

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