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Here’s an overview of #COVID19 testing by @LKucirka, @justinlessler, & co on the relationship between the false negative rate (RT-PCR) & when you were tested. This is a *crucial* graph showing how your *pretest* probability of infection is updated by the test result over time
Let’s focus on one of the lines: the dashed line indicating that the pretest probability is 11%
Context: A recent large study of household contacts estimated the *pretest* probability of infection given someone in your house tested positive is 11.2%
Context: A recent large study of household contacts estimated the *pretest* probability of infection given someone in your house tested positive is 11.2%
Looking at this graph, if
someone in your household tests positive
you are tested on the 1st day of exposure
you test NEGATIVE (phew!)
your probability of being infected *even though you tested negative* is still 11%
WHY? The test is bad at detecting early results
someone in your household tests positive you are tested on the 1st day of exposure you test NEGATIVE (phew!) your probability of being infected *even though you tested negative* is still 11%WHY? The test is bad at detecting early results
Waiting a few days can help, but it doesn’t bring the probability of a false negative to 0.
Here’s the overall relationship between the probability of a false negative result & days since exposure - best case scenario, you’re tested on day 8 (~3 days after symptom onset)
Here’s the overall relationship between the probability of a false negative result & days since exposure - best case scenario, you’re tested on day 8 (~3 days after symptom onset)
Even in this best case scenario, the probability of a false negative test result is twenty percent! Yikes!
So what does this mean?
If you test negative but your pretest probability of infection is *high* to keep yourself & your community safe operate as if you tested positive
If you test negative but your pretest probability of infection is *high* to keep yourself & your community safe operate as if you tested positive
What makes your pretest probability high? Here are a few things:
did someone you are in close contact with test positive?
were you recently in an environment that has a high likelihood of exposure?
do you have symptoms consistent with infection? https://twitter.com/lucystats/status/1260898968512364544?s=21
did someone you are in close contact with test positive? were you recently in an environment that has a high likelihood of exposure? do you have symptoms consistent with infection? https://twitter.com/lucystats/status/1260898968512364544?s=21
Here’s a link to the paper:
https://www.acpjournals.org/doi/10.7326/M20-1495
AND (I love this
) a link to the researchers’ github with all their code & analysis (
thank you!)
https://github.com/HopkinsIDD/covidRTPCR
https://www.acpjournals.org/doi/10.7326/M20-1495AND (I love this
) a link to the researchers’ github with all their code & analysis ( thank you!) https://github.com/HopkinsIDD/covidRTPCR
