Product type: Nucleic acid testing (NAT) technologies that use real-time polymerase chain reaction (RT-PCR) for detection of SARS-CoV-2 Date: 7 December 2020
WHO-identifier: 2020/5, version 1 Purpose of this notice: To ensure users of certain nucleic acid testing (NAT) technologies are aware of certain aspects of the instructions for use (IFU) for all products. Description of the problem: WHO has received user feedback on an elevated risk for false SARS-CoV-2 results when testing specimens using RT-PCR reagents on open systems.
As with any diagnostic procedure, the positive and negative predictive values for the product in a given testing population are important to note. As the positivity rate for SARS-CoV-2 decreases, the positive predictive value also decreases. This means that the probability that a person who has a positive result (SARS-CoV-2 detected) is truly infected with SARS-CoV-2 decreases as positivity rate decreases, irrespective of the assay specificity. Therefore, healthcare providers are encouraged to take into consideration testing results along with clinical signs and symptoms, confirmed status of any contacts, etc.
Source: WHO Information Notice for IVD Users
Well WHO would have thunk it?
The WHO have finally advised on, not only the danger of false positives based on CT cycles but also on the PPV based on prevalence of the infection of SARS-CoV2.
In fact the advice PPV was the first paragraph of their notice. Whilst this is well known amongst health professional, no guidance to those testing ‘positive’ has ever been given by any government.
I have covered this in a previous article The Mathematics of Covid-19 Testing.
It’s a simple matter of Bayesian Probability. Obviously government ministers can’t do mathematics.
Will governments revise their numbers of Covid positive? Aye right!
That's Enough of That! 
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