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The antibody tests are not wrong half the time. They're wrong a very small percentage of the time, but in areas with little COVID that's enough to make testing unreliable.
Say a test is 98% specific, that is it results negative correctly 98% of the time, but 2% of the time is positive in someone who doesn’t have antibody.Furthermore some of these tests being used appear to be around 90% sensitive currently, which means 90% of the time they pick up the people with antibodies and 10% of the time they miss it.Let's say you use your test in an area with 2% COVID antibodies in the population.
So therefore out of 1000 people 20 actually have COVID antibodies. How does your test do?
Your sensitivity is 90%, so you detect 18 real positives. You miss 2, so you have 2 false negatives.
Your test is 98% specific. So you have 980 people without COVID antibodies, so (98% * 980 people without COVID) is 960 negative tests. So 20 real negatives test false positive.
So your totals for this population are:
You have a total of 38 positives.
You have a total of 962 negatives.
Before you adjust for the test, it looks like your prevalence of disease is 3.8%.
However, the positive predictive value of a positive test is less than 50%. Only 18/38 positives truly have COVID antibodies - that's 47%. This is what they mean when the test can be wrong half the time. So if you test positive in this situation, it's slightly worse than a coinflip as to whether you actually have antibody.
What about a negative? You have 2 false negatives and 960 real negatives. So if you have a negative test, you chances of being a true negative are (960 / 962) or 99.7%. So there is a 0.3% chance you tested negative but had antibodies.
So I think we can be pretty confident that @fidoz is actually negative.
What happens with the same test in a high prevalence situation:
If 20% have COVID antibodies out of 1000 people
200 positives. Your test misses 20 of them and 180 are true positives. (200*0.9 sensitivity)
There are 800 negatives, and with your test 784 test negative (800 * 0.98 specific). So you have 16 false positives.
So you have 196 total positives. If you test positive, what does it mean for whether you have the antibodies?
So now there's a 180/196= 91% chance you really do have COVID antibodies, when using the same test as before.
And for the negatives there are 804 total negatives (784 + 20 false negatives). So the chances of a negative being real in this case is
784/804= 97.5% chance.
Note how you'd like your test to be more sensitive and not miss any real positives, because the false negatives are becoming more significant as the number of real negatives drops.Marci said:Antibody tests are not turning out to be reliable at all (wrong about half the time, according to the CDC):https://amp.cnn.com/cnn/2020/05/26/health/antibody-tests-cdc-coronavirus-wrong/index.html?fbclid=IwAR0Op5B170HL--ZJGBb_M7H1D0qNmuKUXYgqyTmM0eywccqjgusB1koHq5Q
Hatorian said:Trubisky was still the right pick at 2....just watch....I don’t care if I’m going down To an icy grave on the Trubanic....haha
Michelle said:I can’t remember who here on the forums recommended the Pack Your Knives podcast, but it’s great! I’ve been listening all season and highly recommend it. Listening to their finale podcast now.
Yeah, they're really good. The interview with Kevin was very interesting, There's a lot I didn't know about that guy:He was not trying to win the concept for Restaurant Wars either, which is why his concept had problems, they weren't supposed to pick it. Also he got into MIT as a nuclear engineer and didn't go?!?
Their interviews with the cheftestants have all been very interesting, I believe the three finalists are coming up this week.
"How often do you hear the writing is lazy, or the writing is bad? You didn't hear that about 24, or Battlestar Galactica or Mad Men, but it's a common complaint about New Star Trek."
Have we considered the same "Old Trek" fans who liked serialized DS9 and possibly even Babylon 5 and possibly even BSG might be able to handle one of these here newfangled serialized Trek shows if they were actually good?
“Nothing Trump is accused of from now on by the press will be believed by huge chunks of the population, a group that (perhaps thanks to this story) is now larger than his original base.“