weiwentg wrote:
Just so nobody misunderstands me, I am
not trying to play devilās advocate with COVID here. However, it is
possible that Gaviria actually got a false positive. All tests may produce false positives. And when you are trying to do screening for a rare outcome, you can get a lot of false positives even with a very good test. Michael Matthews may have been in this situation, since he got an initial positive test and then he returned a couple of negative ones. I recall at least one female pro having a false positive also, but I forget her name.
Thereās not really a way around this aside from re-testing, preferably with a gold standard test if you have one. Right now, I think the PCR test is effectively the gold standard. I would assume that Gaviria will get a follow up test.
If you read the academic literature summarizing the test performance of PCR tests (I.e. sensitivity and specificity), you may see the article discuss this fact, and some articles will probably mention something about the pre-test probability of the disease playing a role in interpreting the test result. This is what that means. If someone showed up in the ER with shortness of breath and other COVID symptoms, and you could rule out a non-COVID cause (e.g. pre-existing COPD), but they tested negative for COVID, you probably would assume thatās a false negative. That personās pre-test probability of Covid is very high. (A synonym may be the prior probability.) Gaviria is the opposite case, in that heās currently asymptomatic, and he has had symptomatic COVID before, so he
should be expected to have some immune response (keeping in mind that there is documented evidence of reinfection, and who knows, maybe heās asymptomatic now
because of that immune response). And all that gets modified by the current situation in Italy, which is worsening (although Spain is worse, so the same situation in the Vuelta would be more likely a true positive).
Basically, itās complicated, and thereās some math involved, but Gaviria could be a false positive. If I had been writing the rules, I would think about allowing someone back in the race if they produced a negative follow up PCR test, although they still need to isolate in the interim. Naturally, he could really be infected also. And no, I donāt know what the ratio of true positives to false positives is, because I donāt know the correct prior probability of having COVID in the Giro dāItalia, nor do I know the actual sensitivity and specificity of the specific PCR test the Giro is using. Also, I assume this one is a PCR test and not a rapid test, which has lower sensitivity and will produce more false negatives. Actually, thereās a case that the Giro could have decided to screen people daily with the rapid tests, and if someone in your team has a positive, the entire team needs to get a PCR test.
I am not sure if I said here that I thought the Giro should have taken Vaughtersā suggestion for an exit plan seriously. I stand by that statement. Right now, it looks like the Giro may be able to make it to Milan in reasonable safety, so Iām happy to be wrong there, but they and the Vuelta do need to think about an exit plan if enough positive tests come up. I am honestly not confident the organizers of either race would be willing to pull the plug if the situation demanded it.
I believe false positives with PCR-tests are rare, so rare that a positive should be treated as such. Maybe if in the next couple of days he would test negative you could allow him back in the race, but that would still mean he would miss a few days, so it's not a possibility. Even if the chance of a re-infection is rare too. Why take the risk, from an organisers point of view?
Kruiswijk tested postive but was asymptomatic, and felt great, however he tweeted a few days ago that he did develop symptoms a few days after the positive test.