The World Health Organization (WHO) said on Saturday that it was looking into South Korea’s report that 91 people who had ostensibly recovered from COVID-19 later tested positive for the virus again.
Not to worry.
We’ve had it completely under control since January.
What’s the false negative rate on PCR tests?
So I guess Donnie is going to blame the WHO for not knowing this about a novel virus that no one knew anything about five months ago and epidemiologists and virologists have been working night and day to analyze.
The test itself has a low false negative rate if a reliable sample was taken. However the latter is not a given; a covid-19 nasal swab needs to be obtained from well back inside the sinus cavity, and it’s all too easy to not go far enough, not swab around enough, or both. The hospital group in which I work has issued revised and increasingly detailed instructions on nasal swab technique, because there was a little too much variability in how these are being collected. (btw, my favorite quote from Cuomo’s press conferences so far, in reference to a forthcoming short nasal swab test that might be easier to administer: “I’m not exactly sure what a short nasal swab test is, but I assume it would not apply to me” (gestures at big Italian nose)
The PCR process itself can detect even small viral loads, so if a viral sample is present on the nasal swab then the PCR false negative rate is very low. Without going too far down the road of science-splainin’, the purpose of a PCR test is to amplify even very small amounts of genetic material, and that means even small infections can and should be detected. A reasonably low-level infection doesn’t magically transfer itself to the nasal swab, however.
US now over 20,000 deaths from COVID-19. 521k cases (actually 581k when you include long term care facilities and US military). On our way to another 30k caseload today.
This is a novel virus with much that we don’t know. That’s what I would tell people anxious to open things up already -prematurely.
Does anyone know how this compares to previous coronavirus infections like SARS or MERS?
I’ve seen false negative rate estimates as high as 30%
A lot depends on tester error.
ETA:
In one scenario, the initial swab sample may not always collect enough genetic material to provide an accurate test. This problem may arise more often in patients who do not show many symptoms at the time of their test, the Globe reported. In addition, the standard nasopharyngeal swab, wherein a long instrument is wriggled and rotated to the very back of the nasal cavity, can be both difficult for clinicians to perform and uncomfortable for patients to endure, Krumholz wrote.
Alternatively, false-negative results may result from how tests are processed in the laboratory or what specific chemicals are applied to each sample, Krumholz noted.
This article assumes 90% for the sake of an argument.
“RT-PCR testing is most useful when it is positive,” says Dr. Sampathkumar. “It is less useful in ruling out COVID-19. A negative test often does not mean the person does not have the disease, and test results need to be considered in the context of patient characteristics and exposure.”
Even with test sensitivity values as high as 90%, the magnitude of risk from false test results will be substantial as the number of people tested grows. “In California, estimates say the rate of COVID-19 infection may exceed 50% by mid-May 2020,” she says. “With a population of 40 million people, 2 million false-negative results would be expected in California with comprehensive testing. Even if only 1% of the population was tested, 20,000 false-negative results would be expected.”
I read an interview from the editor of The Lancet medical journal where he described a conversation with Tedros Adhanom Ghebreyesus in February. According to him, Dr. Ghebreyesus was “close to despair” at that time because the WHO’s warnings were being ignored by several major nations, including the US. Makes it all the more obnoxious that Trump is trying to blame the WHO, of all possible organizations, for inaction.
It’s at least 30%, meaning that if you have an active infection there is a 30% chance that the pcr test will give an erroneous negative result. This is why most hospitals will not consider you Covid negative until you’ve had two negative tests in a row. These Korean patients are most likely a reflection of the high false negative rate rather than re-infections.
AKA a “Roman Nose” as I recall.
On a more serious note, re:
thanks for posting this! It’s great to have TPM members from such diverse and relevant backgrounds!
Exactly! And of course these giants have no problem burying plebeian lilliputians.
This is exactly the problem. If anyone is curious, the nasopharyngeal process for obtaining samples can be viewed at this New England Journal of Medicine video here. With a non-compliant patient, a tentative tester, or both, it’s all too easy to obtain an unreliable sample, and the best PCR test in the world can’t compensate for that.
This is correct. The in vitro analyses of the PCR test show it to be reliable. It is the sample collection that is less than ideal.
Those of us with respiratory problems this Corvid 19 is our worst nightmare.And now its recurring after you
have supposedly been cured of it.DAMN !
Would’t you like to be the one shoving that sucker up Trumpy’s proboscis?
“Sorry Mr. President, due to a shortage of test kits I’m going to have to use a toilet brush…”
