Thread by @michaelmina_lab

@elonmusk Great question! It’s more complex question and depends on why the test is being used - ie: do you want to know if you are currently contagious/risky to others or if you have any remnants of RNA? The difference may sound trivial but it is massive

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@elonmusk If the question is "Am I currently infectious right now when the swab is collected" then on *most* instruments/labs, data suggests a Ct value somewhere around 30 or below is needed. (Different instruments/labs are different - but that's a decent generalization)...

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@elonmusk If the question is "Am I becoming infectious" then even if you have a high Ct value... say 38... then it's important to test again the next day. If you go to 28, then you better stay put, you're likely infectious for the next 5-8 days or so...

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@elonmusk If you stay at ~38 or you went from 32 to 35 or something over a 24 hour period - then most likely you are recovering from infection and no longer infectious. Some people stay positive after infection with Cts in mid-upper 30's for weeks/months.

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@elonmusk If want to know if one is a danger to others RIGHT NOW... a rapid antigen test can be your best friend. If positive, immediately confirm with a second *different* rapid test (antigen / lamp / isothermal) or get a PCR - but it has a long turnaround time so not as useful.

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@elonmusk If you just want to know the question that doctors often ask: "Do I have evidence of a SARS-CoV-2 infection" then look for the RNA with a PCR test or PCR-like test. In this case, a Ct value <40 is usually considered reliable on most tests for "Do I have any virus RNA in me"..

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@elonmusk If you actually get a result with a Ct value of 40 though... that's really REALLY low... and can be a false positive. Many labs/tests have decided to use a cutoff an order of magnitude higher.... choosing a Ct value of 37 or lower to be positive.

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@elonmusk Again though - PCR is tough to interpret. It does NOT = infectious. If <30... generally = contagious-ish. <20 DEFINITELY high virus

More time is in post-infectious PCR+ stage than infectious PCR+... this is where antigen test shines. If +ve and confirmed... means contagious

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@elonmusk Finally - different labs use different tests. @AbbottNews m2000 test for example shaves off the first 10 Ct cycles when reporting. So a 24 on m2000 = 34 cycles. Other tests may use more or less efficient enzymes. But cycle per cycle, they all do a decent job at doubling per cycle

@elonmusk @elonmusk - My quick answers to your question are above. I think you'd be interested in the detailed answer. Send me a DM if you want. You're a scientist and it would be GREAT if you could help inform the world of the right answer to your question - So much confusion abounds.