On the 31st of May this year, an interesting scientific letter from German researchers was published in the ‘Journal of infection’ on the true value of a “positive” RT PCR test. The entire world now knows of the PCR test invented by Kary Mullis, who received the Nobel Prize for his invention. Importantly, the PCR test does not pick up live infective virus. It only detects genes of a virus. The German researchers have used a PCR test developed by Roche , which picks up ORF and envelope genes. They considered a cut off value of 25 cycle threshold  (Ct), which is also the cut off values for home screening by the Office of National Stats (ONS) of England. 


They checked more than 160000 people with the PCR test, and found 4000 positive cases. There have been 3 phases in Germany. 


The mean Ct value was 27.8 in the first phase, 28.8 in the second phase and 26.6 in the third phase. 


When a Ct of 25 was taken as the cut off point for infectivity, positive cases were 27% in the first phase, 27% in the second phase , and 40% in the third phase. This means that 60% to 73% of cases of positive PCR were not infective.


When a cut off of 30 Ct was taken, there were 55% positive cases in the first and second phases, while 75% were positive in the third phase, hence 25% to 45% were not infective cases.


The researchers concluded that positive PCR tests should not be taken as proof of infectivity. A positive test may simply indicate a past infection.



The performance of the SARS-CoV-2 RT-PCR test as a tool for detecting SARS-CoV-2 infection in the population

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