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GidMK's profile
Health Nerd
Health Nerd
Health Nerd
Verified account
@GidMK

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Health NerdVerified account

@GidMK

Epidemiologist. Writer (Guardian, Observer etc). "Well known research trouble-maker". PhDing at @UoW Host of @senscipod Email gidmk.healthnerd@gmail.com he/him

Sydney, New South Wales
theguardian.com/profile/gideon…
Joined November 2015

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    1. Health Nerd‏Verified account @GidMK 1 Jul 2020

      Health Nerd Retweeted Health Nerd

      Thought I'd take a leaf out of @ProfMattFox's book and use some 2x2 tables to illustrate thishttps://twitter.com/GidMK/status/1278468849294168064 …

      Health Nerd added,

      Health NerdVerified account @GidMK
      Now in a population with a 10% prevalence, that's pretty bad. In 1000 people, you miss 20 true positives and get 10 false positives, so you underestimate prevalence substantially
      Show this thread
      1 reply 3 retweets 8 likes
      Show this thread
    2. Health Nerd‏Verified account @GidMK 1 Jul 2020

      So, here's our table. We've got positive and negative results for our test compared with the truth Here, I've plugged in the numbers for a prevalence of 5% (i.e. 5% of people have had COVID-19)pic.twitter.com/LTc08r14p7

      1 reply 0 retweets 2 likes
      Show this thread
    3. Health Nerd‏Verified account @GidMK 1 Jul 2020

      Now, we know that sensitivity is 80.9% and specificity is 98.9%. Plugging those in, we get this tablepic.twitter.com/ahZXTRpx2D

      1 reply 0 retweets 3 likes
      Show this thread
    4. Health Nerd‏Verified account @GidMK 1 Jul 2020

      From this, we can work out the Positive Predictive Value (PPV) = likelihood of a positive test having actually had COVID-19 Negative Predictive Value (NPV) likelihood of a negative test actually not having had COVID-19

      2 replies 0 retweets 0 likes
      Show this thread
    5. Health Nerd‏Verified account @GidMK 1 Jul 2020

      Here's what that looks like for a population prevalence of 5% Of the people who test positive, only 79% actually have the disease. Of the negatives, 99% have never had itpic.twitter.com/45MF4mDplm

      1 reply 0 retweets 2 likes
      Show this thread
    6. Health Nerd‏Verified account @GidMK 1 Jul 2020

      But if we vary the prevalence, the PPV and NPV change a lot! At 1%, PPV = 43% NPV = 100% At 20%, PPV = 95% NPV = 95%pic.twitter.com/VjEm7YGyPS

      1 reply 0 retweets 1 like
      Show this thread
      Health Nerd‏Verified account @GidMK 1 Jul 2020

      What this means is that if you run the test in a population where very few people who've had the disease, MOST of your positive tests will be false positives This means that your prevalence estimate might be double the true one (or more)

      5:30 PM - 1 Jul 2020
      • 1 Retweet
      • 2 Likes
      • Petter Russ Conser Rob England 🦓
      1 reply 1 retweet 2 likes
        1. Health Nerd‏Verified account @GidMK 1 Jul 2020

          If instead you run the test in a population where many people have had COVID-19, you'll underestimate the prevalence by at least 10% Both of these aren't great scenarios

          1 reply 0 retweets 3 likes
          Show this thread
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