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+1 the major advantage of RT-qPCR is it's a massively amplified test. Tiny amounts of virus can be detected. When you're detecting a patient's antibodies to the virus, the patient is producing massive amounts of it. That said, after clearing the infection that production will decline, but still should be detectable at least for a while. Direct detection of viral particles is substantially harder because for N viral particles you basically get O(N) signal (to put it in CS terms). There are some caveats, your readout antibody can generate an exponential signal for example. You still need to be presenting the right part of the capsid and have the right material. In the direct ELISA scenario, you need 2 antibodies. One to bind the capsid to your surface, the other to bind it again and either have a readout attached directly or be bound by yet another antibody with the readout (likely the latter). Fortunately, viral capsids are repetitive so you might be able to use the same antibody for both. You ALSO need those antibodies to have low cross-reactivity with sputum or whatever other rough sample you have. When you are looking for a patient's antibodies, the patient's immune system has already taken care of that for you. This is all tricky, obviously it's not THAT hard, but it's not trivial. RT-qPCR is directly a O(2^N) signal. It's extremely sensitive, requires very little assay development, can be made very specific, can work with little sample prep, so it's a much better front-line, first-to-go test than direct ELISA. Still, we'd all love an ELISA, particularly to patient antibodies. |
[1] https://doi.org/10.1101/2020.03.05.20030502