- This topic has 6 replies, 6 voices, and was last updated 3 weeks, 6 days ago by
.
-
Posts
-
-
-
I heard on the news thatthereare 30% false negatives…….I remember thinking that seemed incredibly high.
Edit: livescience.com/covid19-coronavirus-tests-false-negatives.html
Don’t know about the reliability of the site:
“”Unfortunately, we have very little public data on the false-negative rate for these tests in clinical practice,” Dr. Harlan M. Krumholz, a professor of medicine at Yale University and director of the Yale New Haven Hospital Center for Outcomes Research and Evaluation, wrote in an opinion piece in The New York Times. However, preliminary research from China suggests that the most common type of COVID-19 test, known as a reverse transcriptase polymerase chain reaction (RT-PCR) test, may give false-negative results about 30% of the time.”
-
Well, there’s an internal control in the kit, so if that has worked, and you don’t get a positive test result, you didn’t put any viral RNA in. Maybe that’s because you screwed up the sample collection or the patient has something else.
RT-PCR really is unbelievably sensitive and an extremely robust technology.
If you said there was an unexplained 30% negative result for antibody tests however, that’s a whole different question
-
That maybe the case, but there are an awful lot of false negatives occurring.
In my workplace, we regard them as 70% sensitive at best.
Maybe the swabs are not done correctly, or maybe the viral load in the sick pneumonic patients does not dwell in the nasal or pharyngeal membranes (I’ve heard bronchial alveolar lavages are nearly always positive, when swab pcr’s are not), but at my workplace we have seen several patients who unequivocally had Covid disease, severe enough to require intubation, and initial swabs were negative. Patients who improve are not routinely swabbed again, but those who continue to deteriorate are. In several cases I’ve seen, the second swabs are also negative for Covid, then third or fourth swabs test positive. (I know, that could indicate in-hospital transmission, but as I said, these patients present with a severe Covid clinical picture that could not be any other diagnosis).
I have also had colleagues go off work with Covid symptoms, really clear history and clinical progression, following very high risk exposure to known positive patients, and their swabs have been negative. They are not re-swabbed if they improve.
For my part, if I develop symptoms, I shall decline a swab. I do not see the point – on an individual basis, it can only be used to encourage premature return to work, which is highly irresponsible given any lack of sensitivity, and is only a dangerous distraction. Nurse colleagues (I’m a doctor) say they feel they cannot refuse a swab test – it will go on their record, and if they continue to selr-isolate after a negative result they will be deemed to have taken unauthorised absence. I know nurses have returned before their 7 days isolation because if swab results I would not have trusted (or, indeed, consented to having taken).
Here’s advice I take:
https://www.medscape.com/viewarticle/927800
And other results……
-
Maybe the swabs are not done correctly, or maybe the viral load in the sick pneumonic patients does not dwell in the nasal or pharyngeal membranes (I’ve heard bronchial alveolar lavages are nearly always positive, when swab pcr’s are not)
That’s interesting and my guess (and it is only a guess) is that this is the issue. Presumably quite a lot of people have an upper respiratory infection as part of their presentation, but maybe quite a few don’t, in which case we’re swabbing in the wrong place. By the same logic, maybe patients with pneumonia have a strong mucosal immune reaction with specific secretory IgA but not much of a systemic IgG titre, which means again, we’re looking in the wrong place and they come up negative on some of the antibody tests currently being tested.
-
I’ve heard that the false negative rate is lower in areas with the longest experience of covid19 such as London, compared to other areas only just experiencing higher volumes and that this is down to inefficient sampling/swabbing. Areas which have been swabbing the longest have learnt to really get in and scrape a decent sample whilst areas just starting to test are being too ‘gentle’.
-
-
-
-
-
You must be logged in to reply to this topic.