For those who just want a quick-ish explanation with a bit of detail. If you want a long-ish explanation with a lot of detail see: for a lot more (text & figures)

Diagnostic tests look for whether a person is currently infected. These include

Do NOT confuse an ANTIGEN TEST with an ANTIBODY TEST! Finding a viral antigen (viral “piece”) in someone tells you the virus is in that someone – now. It’s like catching a robber red-handed. But finding an anti-viral antibody in someone is more like finding a fingerprint the robber left on his way out – it tells you someone was infected in the past (or is currently recovering).⠀

Biochemically, antigen tests works similarly to an antibody test, BUT in REVERSE. Antibody tests are used to detect little proteins called antibodies that your body makes to specifically bind to viral parts to help fight off infections and prevent re-infection. Antigen tests look for those viral parts, which are called antigens and are usually parts of viral proteins. So, these tests give you very different information. Antibody tests tell you if someone “has been” infected – in the past or currently (though antibodies don’t show up until later in the infection). Antigen tests tell you if someone is currently infected – they detect the virus itself which is only there when you’re actively infected, so these tests don’t tell you anything about past infections. Antigen tests are thus another form of diagnostic test, joining the group led by the gold standard RT-PCR tests, but not performing quite as well. 

Format-wise, antigen tests are often “lateral flow assays” – strips in a cassette similar to pregnancy tests, but instead of peeing on them, you “snot on them” 

Antigen tests are less sensitive but is able to detect the virus when people are still acutely infected, and the virus is still making all that RNA to make all the proteins it needs to make more of itself and infect more cells. PCR is so sensitive it can also detect remnants of viral RNA past the point at which a person is actually infective – more on that here:   

P.S. Love you dad!

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