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How Accurate are Antibody Tests?

By Caitlyn RN

First, we should probably review the basics of the antibodies our immune system produces in response to invaders. There are 5 types of antibodies, or immunoglobulin; IgM, IgD, IgG, IgA, and IgE. IgM is the main antibody present during primary immune response. IgG is the most common antibody in our immune system and is predominately present in secondary immune response. These are the antibody isotypes, most commonly IgG, tested for with "antibody tests".


The immune system works in a sort of cascade, invaders/toxins trigger the initial cells which communicate with the next and so on and so on until the whole army of good guys has shown up to do their part to stomp out the funk. This cascade occurs in the same way regardless of the invader (well unless you're injecting via vaccine and bypassing natural barriers that trigger the initialresponse, but that's a whole other discussion for another day). I won't get too in depth with this, but understand that *all* foreign invaders trigger the immune system to kick off this domino effect.


So back to the SARS-CoV2 IgG antibody tests. When you have this test done, they take a little of your blood (serum) and expose it to the "SARS-CoV2" antigen. If the IgG antibodies are detectable above a certain level, youre positive and below, negative. So how accurate are these tests? How sure are they? Let's take a look at what the manufacturers have to say about their SARS-CoV2 antibody tests:


False positive results for SARS-CoV-2 IgG assay may occur due to cross-reactivity from pre-existing antibodies or other possible causes


False positive results for IgM and IgG antibodies may occur due to cross-reactivity from pre-existing antibodies or other possible causes


The IgG antibody isotype predominates during secondary immune responses and is the most common circulating antibody in the immune system.


False-positive results may occur in small percentage individuals - preliminary data indicate minimal cross-reactivity between antibodies to SARS-CoV-2 and the commonly circulating coronavirus strains, OC43, 229E, NL63, and HKU1 https://www.childrensmn.org/references/lab/serology/sars-cov-2-igg-antibody.pdf


False positive results for the EUROIMMUN Anti-SARS-CoV-2 ELISA (IgG) may occur due to cross-reactivity from pre-existing antibodies or other possible causes


Positive results may be due to past or present infection with non-SARS-CoV-2 coronavirus strains, such as coronavirus HKU1, NL63, OC43, or 229E


False positive results for Assure COVID-19 IgG/IgM Rapid Test Device may occur due to cross-reactivity from pre-existing antibodies or other possible causes


False positive results for COVID-19 IgG/IgM Rapid Test Cassette (Whole Blood/Serum/Plasma) may occur due to cross-reactivity from pre-existing antibodies or other possible causes


In a cross-validation of 22 assays (lateral-flow tests and ELISAs) to detect IgM and IgG antibodies in patients with COVID-19, a significant number of positive results were also

found in historic sera from the pre–COVID-19 era and from non–SARS-CoV-2 infections

 

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