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> Agree it's trivially easy to do, but as anyone you does a lot of qPCR knows, they're rife with false positives. Honestly we'd be better off having a widely a

> Agree it's trivially easy to do, but as anyone you does a lot of qPCR knows, they're rife with false positives. Honestly we'd be better off having a widely available test that gives 50% false positives. Worst case, a bunch of people quarantine unnecessarily, but we at least catch all the infected. The alternative is what we have now: widespread community transmission with no end in sight.

glenstein 7y ago
This is an interesting tradeoff to consider. Under many many circumstances I could see that rate of false positives being awful and intolerable. In the context of a bona fide pandemic with a massive shortage of desperately needed testing, however, this would be a palatable improvement upon the status quo.
- jjeaff 7y ago
  In the case of a generally mild disease, it's much less of a problem. Usually, the problem with false positives is unnessary, risky treatments and overwhelming the medical system. But in this case, you just need to know whether to self quarantine. You only need treatment if symptoms become severe.
- GarrisonPrime 7y ago
  Until the global economy unnecessarily collapses.
awrence 7y ago
This is a classic statistical fallacy. A test that is 50% accurate serves 0 purpose. You might as well consider coin flipping a valid test. If you tested America with this test you’d end up thinking half the country is infected. And within the ones that tested positive only half would actually be and within the negatives half would be and you would have missed them. A test needs to be materially more accurate than the odds of having the disease to be worth anything. Classic thought experiment: if 0.0001% of the population has a disease and a test is 99% effective and you test positive, what are the odds you have the disease? Answer: 1%. Complimentary thought experiment: if an expensive preventive drug was available in limited supply (for 0.1% of the population only) and the earlier you took it the more preventive it was should you give it blindly to as many positives as you can? Probably not because 99% of that would be going to waste. And you would run out of drugs to cover the actual sick. Only 10% of the sick would end up actually getting the drugs.
- DangitBobby 7y ago
  Isn't the false positive rate the percentage of positives that should have been negative? E.g. if I test 10000 people and 100 of them test positive, but really only 50 of those are sick (and almost none of the other 9900 people are), then I have a false positive rate of 50% and a very useful test. Obviously, this is very different from flipping a coin. And flipping a coin could also have a very high false negative rate.
  - usrusr 7y ago
    It's the rate of actual negatives that are perceived as positives. I make that mistake myself all the time, What helps me avoid it is this: you want a number that is a property of the test method alone, independent of case distributions. A ratio between misidentified positives and identified positives (or true positives) would depend on sample distribution.
  - monktastic1 7y ago
    The metric you're talking about is called the "false discovery rate": https://en.wikipedia.org/wiki/False_discovery_rate https://en.wikipedia.org/wiki/False_discovery_rate I also have a hard time keeping these all straight: https://en.wikipedia.org/wiki/Sensitivity_and_specificity#Application_to_screening_study https://en.wikipedia.org/wiki/Sensitivity_and_specificity#Ap...
- sliken 7y ago
  50% accurate I'd agree is useless. But the parent said 50% false positive, presumably with close to a 0% false negative would be VERY useful and save potentially millions of lives. We need enough tests yesterday or so to avoid a repeat of Italy and a 50% false positive rate (with a very low false negative rate) could help do that.
  - makomk 7y ago
    Even the properly-conducted version of this test has a fairly substantial false negative rate, somewhere around the 30% mark, and if it's done by students using samples that might not be taken correctly on a rigged-together testing setup that's going to get worse. Seriously, you might as well flip a coin.
bryanrasmussen 7y ago
hmm, how many false negatives is it giving?
beojan 7y ago
With a 50% false positive rate, most of your positives are false positive. You might as well just tell everyone who asks for a test that they're infected.
- dependsontheq 7y ago
  From a public health perspective ... this could be a brilliant solution.
  - mnd999 7y ago
    Until they all decide that they’re immune and don’t need to worry about it anymore.
    - fortran77 7y ago
      Exactly. I think many people, upon learning they're positive by asymptomatic would (maybe) wait a week to make sure they don't feel ill, and then go out into the world, potentially spreading the virus with abandon because they're among the group that doesn't get ill from it.
  - lozenge 7y ago
    Until Fox News or the governor of West Virginia blows the illusion away.
    - ramblerman 7y ago
      Huh, I'm lost in this thread. So, We lie to people and tell em they are infected. And then we are angry that FOX or the governor of WV are going to expose our scam? What did I miss...
      - beojan 7y ago
        You could tell them the test came back positive. That's not really a lie.
dguest 7y ago
You also need to consider that when the false positives get over their flu or cold or whatever, they will assume they are immune to COVID.
astrophysician 7y ago
Really? That test is meaningless. It screws up the numbers, and you might as well do a complete quarantine anyway, since you need to not just quarantine but isolate anyone who’s positive. Let’s say there’s 100,000 people in the US with covid right now (like 10x the measured number) FPR =0.5 TPR = 1 Disease rate = 1e5/3.3e8 = 3e-4 P(you have covid| test is positive) = 3e-4/(0.5(1-3e-4) + 3e-4) = 0.06%!! This is less than useless. Now instead of tracking the disease you are completely overwhelmed with false positives and the result is exactly the same — everyone stay home.
kazinator 7y ago
A 50% false positive rate means that, since most people don't have the virus, about 50% of all the tests will come back positive. You just about might as well flip a coin instead.

glenstein 7y ago

This is an interesting tradeoff to consider. Under many many circumstances I could see that rate of false positives being awful and intolerable. In the context of a bona fide pandemic with a massive shortage of desperately needed testing, however, this would be a palatable improvement upon the status quo.

jjeaff 7y ago
In the case of a generally mild disease, it's much less of a problem. Usually, the problem with false positives is unnessary, risky treatments and overwhelming the medical system. But in this case, you just need to know whether to self quarantine. You only need treatment if symptoms become severe.
GarrisonPrime 7y ago
Until the global economy unnecessarily collapses.

awrence 7y ago

This is a classic statistical fallacy. A test that is 50% accurate serves 0 purpose. You might as well consider coin flipping a valid test. If you tested America with this test you’d end up thinking half the country is infected. And within the ones that tested positive only half would actually be and within the negatives half would be and you would have missed them. A test needs to be materially more accurate than the odds of having the disease to be worth anything. Classic thought experiment: if 0.0001% of the population has a disease and a test is 99% effective and you test positive, what are the odds you have the disease? Answer: 1%. Complimentary thought experiment: if an expensive preventive drug was available in limited supply (for 0.1% of the population only) and the earlier you took it the more preventive it was should you give it blindly to as many positives as you can? Probably not because 99% of that would be going to waste. And you would run out of drugs to cover the actual sick. Only 10% of the sick would end up actually getting the drugs.

DangitBobby 7y ago
Isn't the false positive rate the percentage of positives that should have been negative? E.g. if I test 10000 people and 100 of them test positive, but really only 50 of those are sick (and almost none of the other 9900 people are), then I have a false positive rate of 50% and a very useful test. Obviously, this is very different from flipping a coin. And flipping a coin could also have a very high false negative rate.
- usrusr 7y ago
  It's the rate of actual negatives that are perceived as positives. I make that mistake myself all the time, What helps me avoid it is this: you want a number that is a property of the test method alone, independent of case distributions. A ratio between misidentified positives and identified positives (or true positives) would depend on sample distribution.
- monktastic1 7y ago
  The metric you're talking about is called the "false discovery rate": https://en.wikipedia.org/wiki/False_discovery_rate https://en.wikipedia.org/wiki/False_discovery_rate I also have a hard time keeping these all straight: https://en.wikipedia.org/wiki/Sensitivity_and_specificity#Application_to_screening_study https://en.wikipedia.org/wiki/Sensitivity_and_specificity#Ap...
sliken 7y ago
50% accurate I'd agree is useless. But the parent said 50% false positive, presumably with close to a 0% false negative would be VERY useful and save potentially millions of lives. We need enough tests yesterday or so to avoid a repeat of Italy and a 50% false positive rate (with a very low false negative rate) could help do that.
- makomk 7y ago
  Even the properly-conducted version of this test has a fairly substantial false negative rate, somewhere around the 30% mark, and if it's done by students using samples that might not be taken correctly on a rigged-together testing setup that's going to get worse. Seriously, you might as well flip a coin.

bryanrasmussen 7y ago

hmm, how many false negatives is it giving?

beojan 7y ago

With a 50% false positive rate, most of your positives are false positive. You might as well just tell everyone who asks for a test that they're infected.

dependsontheq 7y ago
From a public health perspective ... this could be a brilliant solution.
- mnd999 7y ago
  Until they all decide that they’re immune and don’t need to worry about it anymore.
  - fortran77 7y ago
    Exactly. I think many people, upon learning they're positive by asymptomatic would (maybe) wait a week to make sure they don't feel ill, and then go out into the world, potentially spreading the virus with abandon because they're among the group that doesn't get ill from it.
- lozenge 7y ago
  Until Fox News or the governor of West Virginia blows the illusion away.
  - ramblerman 7y ago
    Huh, I'm lost in this thread. So, We lie to people and tell em they are infected. And then we are angry that FOX or the governor of WV are going to expose our scam? What did I miss...
    - beojan 7y ago
      You could tell them the test came back positive. That's not really a lie.

dguest 7y ago

You also need to consider that when the false positives get over their flu or cold or whatever, they will assume they are immune to COVID.

astrophysician 7y ago

Really? That test is meaningless. It screws up the numbers, and you might as well do a complete quarantine anyway, since you need to not just quarantine but isolate anyone who’s positive. Let’s say there’s 100,000 people in the US with covid right now (like 10x the measured number) FPR =0.5 TPR = 1 Disease rate = 1e5/3.3e8 = 3e-4 P(you have covid| test is positive) = 3e-4/(0.5(1-3e-4) + 3e-4) = 0.06%!! This is less than useless. Now instead of tracking the disease you are completely overwhelmed with false positives and the result is exactly the same — everyone stay home.

kazinator 7y ago

A 50% false positive rate means that, since most people don't have the virus, about 50% of all the tests will come back positive. You just about might as well flip a coin instead.