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Lately I'm reading about the good Coronavirus-Tests in South Korea, the faulty tests in the USA and the tests in Indonesia which are according to some Singapore newspapers not even working.

I wonder why these tests are so different and can't find proper information on what which country is testing for.

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First of all the disclaimer: I'm analytical chemist but not clinical chemist. That is, I professionally do things that are related/to some extent similar to test kit development, but not for medical use, and I've only been very remotely involved with microbiology-related testing. OTOH, I expect that pretty much everyone who works in this field is likely to work a lot these days and not to have any time to hang out on SX.

So this is all AFAIK, and I'm happy be corrected - and OP please take this with the appropriate caution against stuff written in the internet by random strangers.

  • The tests are developed in much haste, and undergo some emergency validation rather than the full validation procedure that is usually required. This naturally leads to some uncertainty about their performance, but we accept that in order to have tests available at all right now.
    See e.g. what the FDA says about Covid tests
  • List with FDA Emergency Use Approved Covid tests.

  • Different tests search for different parts of the virus RNA.
    (This is getting very far from my field, but is this the information you're looking for as what exctly is tested [by the CDC-developed test]?)

  • Viruses mutate, so a test that works well at some place at one time may not work as well somewhere else or a few weeks or months later.

  • For a test to work,

    • sampling
    • possibly transport
    • extraction from the sample (difficulty varies e.g. between mouth swabs and sputum)
    • the test kit

    must all work well. E.g. if the sampling is not done properly because there's no properly trained staff available, nothing can be done further down the analytic process. I don't know why the Indonesian tests don't work, but it may be as "simple" as samples undergoing long transportation to a lab with the appropriate equipment so they spoil on the way.

  • The faulty tests in the US seem to have had a wrong reagent - and burocratic approval was needed for the labs to replace that by their own good reagents.
    (Usually, if a lab wants to use other than the prescribed/delivered reagent, they need to a lengthy validation of the procedure - the point here is to be allowed to avoid that as it would take too long)

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