Two things I didn’t know until today: The best test for the presence of SARS-CoV-2, the RT-PCR test, has a false negative problem; and you can be sick from Covid-19 for months (though the average case clears or worsens in 2 weeks).
I. First, false negatives (click the thumbnail for a larger image). If you are tested with the “gold standard” methodology — a deep nasal swab wielded by a trained medical professional, followed up by lab analysis — and your test comes back negative a day or two later, that negative result has in the best case a 1 in 5 chance of being wrong.
The worst time to get tested, in terms of the likelihood of a false negative result, is just after infection and before symptoms show up. In that period, averaging 5 days, the chances of a negative result being incorrect range from 2/3 to certainty. There just isn’t enough virus yet to assure a solid test result — though you are probably silently infectious for some of that time.
So given the small penetration of this viral infection in the population, a negative result on an RT-PCR test is far from definitive and can be nearly meaningless.
These results come from a short, peer-reviewed paper in the Annals of Internal Medicine. The authors performed a “pooled analysis,” using previously published results as input, to arrive at rough estimates of the probability that a negative RT-PCR test result reflects reality. The data are thin: the blue diamonds in the illustration represent the only data points the researchers provide.
II. The second thing I learned today: Covid-19 illness in some individuals can linger far longer than the average of 14 days you have probably read about. As governments currently collect statistics, these victims fall through the cracks: they are neither recovered nor dead.
These “long-haulers,” as some are calling themselves in the online support groups that have sprung up on Facebook, Slack, and Reddit, continue to have recurring symptoms one, two, even three months after they first fell ill. While most avoid the ICU and intubation — and some actually test negative for the virus — see I. above — their lives are anywhere from disrupted to upended. Some can’t stand up in the shower or read a few pages in a book.
[ Note added 2020-06-17: ] For a good close look at some of the reasons behind the problem of false negatives in testing, see this article in the SF Chronicle: Why some people get coronavirus symptoms, but still test negative.
[ Note added 2020-06-22: ] The Atlantic’s Sarah Zhang posted a piece yesterday titled What a Negative COVID-19 Test Really Means.