https://abs.twimg.com/emoji/v2/... draggable="false" alt="🧪" title="Test tube" aria-label="Emoji: Test tube"> 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%

https://abs.twimg.com/emoji/v2/... draggable="false" alt="🏡" title="House with garden" aria-label="Emoji: House with garden"> 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

https://abs.twimg.com/emoji/v2/... draggable="false" alt="🏡" title="House with garden" aria-label="Emoji: House with garden"> someone in your household tests positive
https://abs.twimg.com/emoji/v2/... draggable="false" alt="📆" title="Tear-off calendar" aria-label="Emoji: Tear-off calendar"> you are tested on the 1st day of exposure
https://abs.twimg.com/emoji/v2/... draggable="false" alt="🧪" title="Test tube" aria-label="Emoji: Test tube"> you test NEGATIVE (phew!)
https://abs.twimg.com/emoji/v2/... draggable="false" alt="🦠" title="Microbe" aria-label="Emoji: Microbe"> 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)

Even in this best case scenario, the probability of a false negative test result is twenty percent! Yikes!

So what does this mean?

https://abs.twimg.com/emoji/v2/... draggable="false" alt="🧪" title="Test tube" aria-label="Emoji: Test tube"> 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:

https://abs.twimg.com/emoji/v2/... draggable="false" alt="👉" title="Right pointing backhand index" aria-label="Emoji: Right pointing backhand index"> did someone you are in close contact with test positive?
https://abs.twimg.com/emoji/v2/... draggable="false" alt="👉" title="Right pointing backhand index" aria-label="Emoji: Right pointing backhand index"> were you recently in an environment that has a high likelihood of exposure?
https://abs.twimg.com/emoji/v2/... draggable="false" alt="👉" title="Right pointing backhand index" aria-label="Emoji: Right pointing backhand index"> do you have symptoms consistent with infection? https://twitter.com/lucystats/status/1260898968512364544?s=21">https://twitter.com/lucystats...

Here’s a link to the paper:

https://abs.twimg.com/emoji/v2/... draggable="false" alt="🔗" title="Link symbol" aria-label="Emoji: Link symbol"> https://www.acpjournals.org/doi/10.7326/M20-1495

AND">https://www.acpjournals.org/doi/10.73... (I love this https://abs.twimg.com/emoji/v2/... draggable="false" alt="🥰" title="Smiling face with 3 hearts" aria-label="Emoji: Smiling face with 3 hearts">) a link to the researchers’ github with all their code & analysis (https://abs.twimg.com/emoji/v2/... draggable="false" alt="👏" title="Clapping hands sign" aria-label="Emoji: Clapping hands sign">https://abs.twimg.com/emoji/v2/... draggable="false" alt="👏" title="Clapping hands sign" aria-label="Emoji: Clapping hands sign">https://abs.twimg.com/emoji/v2/... draggable="false" alt="👏" title="Clapping hands sign" aria-label="Emoji: Clapping hands sign"> thank you!)

https://abs.twimg.com/emoji/v2/... draggable="false" alt="🔗" title="Link symbol" aria-label="Emoji: Link symbol"> https://github.com/HopkinsIDD/covidRTPCR">https://github.com/HopkinsID...