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LittleDiva
LIF Adult
Member since 9/11 1284 total posts
Name:
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PCR tests and the WHO
I’ve been seeing info all over about the PCR tests and false positives. I’m not in the medical field so many things I read confused me more. But this article, although claimed it to be false had this interesting statement in it. Could this be why someone with no symptoms or mild symptoms tests negative on a rapid and positive on a PCR? Is a rapid really not that inaccurate?
“That’s why the WHO notice urges lab technicians to view weak positive test results (ones with high cycle thresholds) with caution and to consult the instructions on their PCR machinery before issuing positive or negative test results”
Can anyone explain this claim in a way that someone with no medical knowledge can understand. Because I’m leaning towards PCR gives false positives.
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Posted 1/23/21 10:29 AM  |
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PitterPatter11
Baby Boy is Here!
Member since 5/11 7612 total posts
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Re: PCR tests and the WHO
Posted by LittleDiva
I’ve been seeing info all over about the PCR tests and false positives. I’m not in the medical field so many things I read confused me more. But this article, although claimed it to be false had this interesting statement in it. Could this be why someone with no symptoms or mild symptoms tests negative on a rapid and positive on a PCR? Is a rapid really not that inaccurate?
“That’s why the WHO notice urges lab technicians to view weak positive test results (ones with high cycle thresholds) with caution and to consult the instructions on their PCR machinery before issuing positive or negative test results”
Can anyone explain this claim in a way that someone with no medical knowledge can understand. Because I’m leaning towards PCR gives false positives.
The rapid and the PCR are looking at two different aspects of the virus.
The rapid test is looking for specific antigens (proteins on the membrane). The problem with the rapid test is: you test what you get. Poor sample quality leads to false negatives. People complain about the swab going all the way up their nose, but that is how they get good samples. If you get a negative result from the antigen test and have symptoms of Covid, you should also have your sample sent out for PCR testing and isolate while awaiting results. The rapid test is just not as sensitive to low viral loads.
Here is a good illustration:
PCR is a process that copies mRNA in a small test tube using series of heating and cooling cycles to replicate (copy) the genetic information. *If present,* the mRNA increases exponentially after each cycle.
1-->2-->4-->8-->16-->32-->64-->ect.
Threshold count is the number of cycles that are run to multiple the viral RNA until it gets to a point that it reacts and is considered positive.
A higher threshold counts indicates a lower viral load because it needs to be copied more times to get a positive reading.
The problem is if someone has a high threshold count, you can not tell if that person is at the beginning or end of their infection.
Tests with a weak positive should be repeated.
If the threshold count goes up, then they are at the end of their infection and may not require quarantining for 10 days.
If the threshold count goes down, then they are at the beginning of their infection.
PCR can not amplify what is not there. These people are still cases. Even if a person is at the end of their infection, they still count as a case.
PCR false positives are rare and due to lab error. I am sure many here have not done PCR before - I have - although not extensively. It is a tedious process and mistakes can happen. These lab technicians follow rigorous SOP (standard operating procedures) to ensure that samples are not cross contaminated.
Message edited 1/23/2021 1:52:07 PM.
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| Posted 1/23/21 1:44 PM |
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LittleDiva
LIF Adult
Member since 9/11 1284 total posts
Name:
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Re: PCR tests and the WHO
That picture explains a lot. Thank you.
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| Posted 1/23/21 2:24 PM |
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