What is the PCR Test for SARS-CoV-2 and how reliable is it?

What is the PCR Test for SARS-CoV-2 and how reliable is it?

The Reverse Transcriptase-Polymerase Chain Reaction test (RT-PCR) has been used in Canada as the ‘Gold Standard’ test for detection of the SARS-CoV-2 virus. It has some limitations and concerns although safety is not one of them, because this test is performed in a lab with precautions.

Briefly, the PCR test takes a portion of the RNA strand (rather than the whole virus), makes a DNA copy, and then duplicates it with each cycle. There is a fluorescent marker attached to each duplicated template. The more virus that is present in the beginning will require fewer duplication cycles to observe the fluorescence. To give one a perspective on the amplification process, consider the math. One amplification cycle produces two copies. The second cycle produces four copies. The approximate number of copies after 24 cycles is 16 million, 33 cycles is 8.5 billion and 40 cycles is one trillion copies.

The link below to the Ontario Public Health article explaining the RT-PCR test identifies the 27th cycle (128 million copies) as the control cycle. This means that they have taken a known positive sample and amplified it 27 times to demonstrate a positive sample. This means that if the original sample of the virus was used to infect isolated human cells in culture plates, this would be evident. Ontario often uses cycle thresholds of 38 to 40. This will include many false negative results as well as results that are so weak (asymptomatic people) that they are of no concern, except if the goal is to demonstrate high numbers of cases to fuel community anxiety. For example, fragments of dead virus in the air and even in waste sewage can yield positive results at high cycle numbers.

Here is a link to Ontario Public Health’s explanation-

https://www.publichealthontario.ca/-/media/documents/ncov/main/2020/09/cycle-threshold-values-sars-cov2-pcr.pdf?la=en

Rapid antigen tests done at home or in the workplace can soon be used to screen for positivity within 15 minutes. This would alert people to the need for isolation measures and early treatment opportunities. Other tests will determine if the person has antibodies or shows natural, long term immunity due to previous exposure. This knowledge would help us all toward a goal of herd immunity without solely being dependent on vaccines of limited range and unknown time of effectiveness.

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