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With my work we're required to get frequent COVID tests. Sometimes it happens we get COVID tested twice before the first day of work. So for example if I'm working Saturday I may get tested the previous Saturday and Thursday. Wouldn't the most recent test be the most accurate? What's the point of doing two? Even if COVID is dormant in the body, wouldn't the second one still be more accurate? There's a lot of bureaucracy in the industry and it wouldn't surprise em if this practice is pointless.

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Multiple tests can decrease the chance of a false negative.

The probability of detection varies with the time since exposure. This Nature article presents an informative (albeit rough) graph. Suppose, for example, your Saturday and Thursday tests occurred at the times I labeled on the graph below:

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Then it would be quite possible for the Saturday test to return a (false) negative result while the Thursday test returns a positive result, even though both were testing for the same infection.

Notice (in the scenario above) that both tests occurred before the onset of symptoms, so it would be impossible to know if you were infected without testing.

Both tests are valuable since detecting infections as early as possible reduces opportunities for the virus to spread (assuming the infected person quarantines themself).

The second test may not always be the more accurate. With an antigen test, for example, it is conceivable that the Saturday test could occur near the peak of the orange curve (when probability of detection is most likely), and the Thursday test then occur when the probability of detection is much lower.

Moreover, keep in mind that the graph is only depicting probabilities. If the Saturday test has, say, a 40% chance of detecting the virus, but the Thursday test has an 80% chance, then there is still an 8% chance (= 0.40 * (1 - 0.80)) that if the person is infected, the Saturday test returns a true positive while the Thursday test returns a false negative.

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  • Multiple tests will also of course increase the chance of a false positive. – rhialto Oct 31 '20 at 20:30
  • While multiple tests increase the likelihood that some test returns a false positive, the composite result of multiple tests is less likely to be a false positive. This is why the CDC says after testing positive you can end your isolation earlier if you receive 2 negative tests in a row done 24 hours apart. – unutbu Oct 31 '20 at 20:55

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