SARS-CoV-2 PCR Testing Scrutinized for Returning Variable Results

Disclaimer: This article is not considered medical advice. Consult your doctor.

The Polymerase Chain Reaction (PCR) test, invented in 1985 by Kary Mullis, is one of the methods being used by laboratories to detect SARS CoV-2 in patients, and can deliver various results depending on their use.

The PCR test duplicates a tiny amount of a patient’s DNA or RNA into large detectable quantities, by running samples through amplification “cycles.” Too few cycles eliminate valid results, and too many cycles increases false positives. (Some tests, like those from Thermo Fisher recommend 25-30 cycles.)

Canadian researcher David Crowe, with a degree in biology and mathematics, explains, “So, if you cut off at 20 [cycles], everybody would be negative. If you cut off a 50, you might have everybody positive… In one paper I found 37 cycles. If you didn’t get enough fluorescence by 37 cycles, you are considered negative. In another, paper, the cutoff was 36. Thirty-seven to 40 were considered ‘indeterminate.'”

Laboratories set their own cycle thresholds. The CDC has suggested that above 33 cycles it is difficult to detect any virus. Also, too high a threshold may detect genetic fragments that pose no risk.

According to Angela Rasmussen, a virologist at Columbia University, “It’s just kind of mind-blowing to me that people are not recording the C.T. [cycle thresholds] values from all these tests — that they’re just returning a positive or a negative.”

A widely circulated article by Torsten Engelbrecht and Konstantin Demeter has cited evidence that PCR tests are scientifically meaningless for SARS CoV-2, which Politifact has debunked, which prompted a follow-up refutal.

Infographic courtesy the Mayo Clinic.


PCR (Polymerase Chain Reaction)
Confirms if you have the remains of a virus in your body. (Not antibodies)

Antibody tests
Determines if you have been infected with SARS-CoV-2 in the past. (Antibodies start developing within 1 to 3 weeks after infection, but could disappear months later if case was mild.)

Antigen tests
Identifies virus mid-infection as it’s multiplying. The Trump Administration has purchased 150 million from Abbott Labs. (Antigens are proteins on the outside of the virus.)