Another cool data point! I found a paper from Singapore, Jul 2020, testing tear swabs but incidentally giving a bunch of PCR tests too. I’m much more likely to trust a paper that gives PCR tests incidentally, rather than is directly testing their effectiveness with researcher bias toward better results. By counting up the squares by hand, this paper shows 24⁄108 PCR tests came back negative if I counted correctly: that’s 22% false negative rate (FNR).
Now, for adjustments:
First, these patients were recruited from a hospital. So they obviously have much higher viral load than the average person, so we’d expect higher FNR for the general population. (And we see the expected relationship between viral load and positive results: people with average low Ct values (meaning high viral load) rarely test negative, but those testing negative lots have very high Ct on their positive tests.)
On the other hand, only 2⁄17 patients test negative >50% of the time; a lot of the negatives come near the end of a patient’s sickness or hospital stay. So we don’t see great empirical evidence for the hypothesis that some people are consistent false-negatives. If you take out the negatives-at-the-end effect, there are far fewer false negatives, maybe 5-10%. However, this is basically moot because of the selection effect for the hospitalized as mentioned above. Of course you’ll see hardly any consistent-false-negative-patients in the hospitalized!—the fact you see any macroscopic number of false negatives in the middle of progression is a terrible sign (and, if there were any fully-false-negative patients, we wouldn’t see them anyways! Bad filter).
And we do see the requisite theoretical evidence. Because of the two patients with repeated false negatives and low viral load when positive, we can easily extrapolate that some patients just have slightly lower viral load and test negative consistently.
Overall, there isn’t much easy way to convert this study into “FNR on asymptomatic individuals who get tested”. However, I think if 5-10% of tests on the hospitalized came back negative, that strongly implies more than a 20% FNR on the asymptomatic. I would personally guess that this lends credence toward 10-40% FNR on the symptomatic and 20-80% FNR on asymptomatic. (Lest I double-count evidence, let it be known I’m basing these numbers in part on the above analysis of my personally-known symptomatic individuals with ~40% FNR.)
Another cool data point! I found a paper from Singapore, Jul 2020, testing tear swabs but incidentally giving a bunch of PCR tests too. I’m much more likely to trust a paper that gives PCR tests incidentally, rather than is directly testing their effectiveness with researcher bias toward better results. By counting up the squares by hand, this paper shows 24⁄108 PCR tests came back negative if I counted correctly: that’s 22% false negative rate (FNR).
Now, for adjustments:
First, these patients were recruited from a hospital. So they obviously have much higher viral load than the average person, so we’d expect higher FNR for the general population. (And we see the expected relationship between viral load and positive results: people with average low Ct values (meaning high viral load) rarely test negative, but those testing negative lots have very high Ct on their positive tests.)
On the other hand, only 2⁄17 patients test negative >50% of the time; a lot of the negatives come near the end of a patient’s sickness or hospital stay. So we don’t see great empirical evidence for the hypothesis that some people are consistent false-negatives. If you take out the negatives-at-the-end effect, there are far fewer false negatives, maybe 5-10%. However, this is basically moot because of the selection effect for the hospitalized as mentioned above. Of course you’ll see hardly any consistent-false-negative-patients in the hospitalized!—the fact you see any macroscopic number of false negatives in the middle of progression is a terrible sign (and, if there were any fully-false-negative patients, we wouldn’t see them anyways! Bad filter).
And we do see the requisite theoretical evidence. Because of the two patients with repeated false negatives and low viral load when positive, we can easily extrapolate that some patients just have slightly lower viral load and test negative consistently.
Overall, there isn’t much easy way to convert this study into “FNR on asymptomatic individuals who get tested”. However, I think if 5-10% of tests on the hospitalized came back negative, that strongly implies more than a 20% FNR on the asymptomatic. I would personally guess that this lends credence toward 10-40% FNR on the symptomatic and 20-80% FNR on asymptomatic. (Lest I double-count evidence, let it be known I’m basing these numbers in part on the above analysis of my personally-known symptomatic individuals with ~40% FNR.)