It also requires more PPE. This is usually done if you are presumptively positive and the nurse, doctor, whoever needs to be suited up. It is not something that lends itself to being self-administered. A blood stick test requires much less PPE and could be self-administered. There will be a lot of blood stick tests done on presumptively negative members of the population to determine who are asymptomatic carriers or who had it and didn't know it.
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