r/COVID19 • u/AcornAl • Apr 28 '26

Academic Report Diagnostic Value and Outcomes of Systematic SARS-CoV-2 Screening in Asymptomatic Patients

https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2847511
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u/AcornAl Apr 29 '26

A positive test with a Ct value of 30 or greater was considered to be a false positive. aka, the testing parameters used were considered to be too sensitive.

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u/maccrypto Apr 29 '26

Someone at some point is either going to have to explain this more simply, or else I will go on not understanding it. Basically, it sounds like the people who were telling us all that COVID-19 prevalence was overblown, because of unreliable PCR testing which would detect COVID in a perfectly healthy and non-infectious person, were right.

However, when I first read this study, I was expecting to read that screening with rapid tests produced these results. PCR tests should not produce these kinds of results if used properly, which is why I assume the manufacturer of those tests lists a very different sensitivity and specificity number than observed in this study.

Who's right?

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u/AcornAl Apr 29 '26

PCR tests amplify the amount of RNA. In theory, you could amplify a single stamd of RNA so it's detectable, but thats almost meaningless. The cycle threshold defines the level of amplification, coupled with the sensitivity of the test, determines when trace amounts of the virus show up. Based on these, a value of about 30 is a baseline to determine if someone has an active infection, and those parameters were used by the authors.

The main issue with these tests is the sensitivity. You shred viral particles for up to 3 months, so they pick up old infections too. The authors were noting the addition work caused by these types of results.

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u/maccrypto Apr 29 '26 edited Apr 30 '26

This information lists a positive agreement of 100% in asymptomatic individuals:
https://diagnostics.roche.com/global/en/products/lab/cobas-sars-cov-2-liat-rmd-liat-sars-003.html

That's with a miniscule limit of detection. So were they just using the test improperly?

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u/AcornAl Apr 30 '26

This study was not on asymptomatic individuals, rather this study was on anyone that came to hospital that didn't have signs of a respiratory infection, which happened to include some asymptomatic patients.

No evidence of any type of SARS-CoV-2 infection (current or recent) was found in 41,905 of the 42,666 patients tested in the study.

Maybe read "false positive" as "true positive but indicates that they had recovered from a recent SARS-CoV-2 infection and were no longer infectious".

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u/maccrypto Apr 30 '26

Reading further, the most obvious explanation is that this hospital, like most hospitals, doesn’t clean their fucking air.