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**Hugh** · 14-07-2020, 10:58

https://www.biotechniques.com/corona...-for-covid-19/

Quote:

However, new research from Johns Hopkins University (MD, USA) has found that the chance of these tests giving a false negative – stating no infection when the individual actually is infected – is greater than 1 in 5, at times being far higher. The study, which analyzed seven previously published studies that evaluated RT-PCR performance, calls into question the accuracy of the predictive value of such tests.

Publishing their results in the Annals of Internal Medicine, the researchers stress the need for caution in interpreting any negative results of RT-PCR diagnostic tests, as many other factors, such as the timing of the test, appear to play a role in the accuracy of the results. The probability of a false negative COVID-19 test decreased from 100% on Day 1 of the infection to 67% on Day 4. This further decreased to 20% on Day 8, 3 days after a patient would first start to experience COVID-19 symptoms.

Day 8 appeared to be the optimal time for testing, as after this the probability of a false negative once again began to increase. A 21% probability on Day 9 increased to 66% if testing occurred on Day 21 of infection.

There are likely various reasons for the false negative results. In an interview with the Huffington Post UK, James Gill (University of Warwick, UK) stated he believed that sampling errors may play a part, and some testing centers may be swabbing incorrectly; “It’s a swab that requires some experience to do well, certainly without being uncomfortable,” Gill explained.

As shown in the recent study, timing is also a major factor as the virus moves around the body throughout the course of the infection. “The longer you have the condition, the virus migrates and it goes down into your lungs,” commented Gill. “So even though you’re coughing, you’ve got the fever, we might not be able to get the swab because it might not be there anymore. It might’ve gone down to the lungs. That’s one reason we can miss it.”

A similar study, published earlier this month in the BMJ, also highlighted the inaccuracies of PCR-based testing. Noting that a positive test should hold more weight than a negative one due to the test’s high specificity yet moderate sensitivity. The authors suggested that a single negative test should not be used to rule out infection – particularly in patients displaying symptoms of COVID-19.

When I took part in a Rhinovirus medication trial, you were swabbed twice a day for 10 days, with the swab being done by a doctor, as it requires quite precise placement. It was a very strange feeling, like someone was tickling/scratching inside your head (made your eyes water).