Are test errors going to be highly correlated? If you take two tests (either of the same type or of different types) and both come back negative, how much of an update is the second test?
I’m not super sure; I wrote about this a little in the section “What if you take multiple tests?”:
If you get a false negative because you have a low viral load, or because you have an unusual genetic variant of COVID that’s less likely to be amplified by PCR*, presumably that will cause correlated failures across multiple tests. My guess is that each additional test gives you a less-significant update than the first one.
*This scenario is just speculation, I’m not actually sure what the main causes of false negatives are for PCR tests.
but that’s just a guess. I’d love to hear from anyone who has a more detailed understanding of what causes failures in NAATs and antigen tests.
Naively, I’d expect that if the test fails due to low viral load, that would probably cause correlated failures across all tests taken on the same day. Waiting a few days between tests is probably a good idea, especially if you were likely to be in the early-infection stage (and so likely low viral load) during your first test. The instructions for the BinaxNOW rapid antigen test say that if you get a negative result, you shouldn’t repeat the test until 3 days later.
Yes, accuracy in antigen tests seems to correlate very strongly with viral load (and presumably therefore with infectivity). This paper found 100% agreement with PCR for Ct 13-19.9 (massive viral load), all the way down to 8% agreement for Ct 30-35.
Ct (cycle time) measures how many amplification cycles were needed to detect nucleic acid. Lower Ct values indicate exponentially more nucleic acid than higher values, although Ct values are not standardized and can’t be directly compared between testing facilities.
And same question for a positive test: if you get a positive and then retest and get a negative, do you have a sense of how much of an overall update you should make? I’ve been treating that as ‘well, it was probably a false positive then’, but multiplying the two updates together would imply it’s probably legit?
Yeah, based on the Cochrane paper I’d interpret “one positive result and one negative result” as an overall update towards having COVID. In general, both rapid antigen tests and NAATs are more sensitive than they are specific (more likely to return false negatives than false positives.)
Though also see the “Caveats about infectiousness” section, which suggests that NAATs have a much higher false positive rate for detecting infectiousness than they do for detecting illness. I don’t have numbers for this, unfortunately, so I’m not sure if 1 positive NAAT + 1 negative NAAT is overall an update in favor or away from infectiousness.
Are test errors going to be highly correlated? If you take two tests (either of the same type or of different types) and both come back negative, how much of an update is the second test?
I’m not super sure; I wrote about this a little in the section “What if you take multiple tests?”:
but that’s just a guess. I’d love to hear from anyone who has a more detailed understanding of what causes failures in NAATs and antigen tests.
Naively, I’d expect that if the test fails due to low viral load, that would probably cause correlated failures across all tests taken on the same day. Waiting a few days between tests is probably a good idea, especially if you were likely to be in the early-infection stage (and so likely low viral load) during your first test. The instructions for the BinaxNOW rapid antigen test say that if you get a negative result, you shouldn’t repeat the test until 3 days later.
Yes, accuracy in antigen tests seems to correlate very strongly with viral load (and presumably therefore with infectivity). This paper found 100% agreement with PCR for Ct 13-19.9 (massive viral load), all the way down to 8% agreement for Ct 30-35.
Ct (cycle time) measures how many amplification cycles were needed to detect nucleic acid. Lower Ct values indicate exponentially more nucleic acid than higher values, although Ct values are not standardized and can’t be directly compared between testing facilities.
And same question for a positive test: if you get a positive and then retest and get a negative, do you have a sense of how much of an overall update you should make? I’ve been treating that as ‘well, it was probably a false positive then’, but multiplying the two updates together would imply it’s probably legit?
Yeah, based on the Cochrane paper I’d interpret “one positive result and one negative result” as an overall update towards having COVID. In general, both rapid antigen tests and NAATs are more sensitive than they are specific (more likely to return false negatives than false positives.)
Though also see the “Caveats about infectiousness” section, which suggests that NAATs have a much higher false positive rate for detecting infectiousness than they do for detecting illness. I don’t have numbers for this, unfortunately, so I’m not sure if 1 positive NAAT + 1 negative NAAT is overall an update in favor or away from infectiousness.