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Frank Bures: False positive COVID PCR tests sow confusion

Frank Bures: False positive COVID PCR tests sow confusion

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Any medical test for anything is not infallible. We have heard in the last 4-6 weeks some interesting results from COVID tests in people close to us. The most recent was an asymptomatic person intending to come to our house for a pass through distanced greeting. That didn’t happen because his saliva polymerase chain reaction or PCR test was positive, but those of his wife and children were negative. He subsequently had two more of the same tests that Minnesota is offering at certain centers from a company called Vault. They were both negative.

Another situation was a special education teacher in class. One student was coughing for a week and produced a positive test. The teacher tested positive for COVID and did get fairly sick with fever, etc., but recovered completely (happily). Her husband got sick as well, had 3 negative tests, not sure what variety, had some symptoms and took longer to recover. Their two teenage sons have been fine. They are distance learning. The family chose not to have them tested.

Another situation was a young fellow who works for a heating/air-conditioning company, and goes into homes daily to rescue folks from freezing when their furnaces crap out (technical HVAC term). His company requires employees to get a nasal swab test every 2 weeks. His have all been negative. Just recently he had a blood test for antibodies to the virus, which was positive. He has never had one headache, fever or cough.

I have seen it written that anywhere from 25% to 40% of people with COVID infections are totally asymptomatic. There are dozens of tests for the virus in use after getting emergency use authorization, or EUA. They broadly fall into three categories.

First are molecular tests also called PCR, viral RNA or nucleic acid tests. These all try to identify the genetic material of the actual virus in the sample which can come from throat swabs, saliva, other body fluids, or nasal swabs, which one cartoon in the Winona Daily News compared to a nasal colonoscopy.

Second are ones trying to find traces of the viral protein envelope’s so-called spike protein, which binds to human cells. These are antigen tests from nasal or throat swabs. And third are the serum tests (from blood) for antibodies against the virus as evidence of past infection.

Here is a discussion from a Dec. 16, 2020, Harvard Medical School Harvard Health education newsletter, which discusses problems of all tests. “The true accuracy of tests for COVID-19 is uncertain. We don’t have precise measures of accuracy for these tests, just some commonly quoted figures for false negatives or positives. How carefully a specimen is collected [!!] and stored may affect accuracy. Because these tests are available by EUA, the usual rigorous testing and vetting [by veterinarians?] has not happened yet. A large and growing number of labs and companies [with different methodologies] offer these tests, so accuracy may vary. We don’t have a definitive ‘gold standard’ test with which to compare them.”

The topic of false negative tests for both PCR and antigen is too much for this small space. False positives have been said to be almost rare for PCR nasal swabs. For the first patient above the Vault test specimen was spit. How much is in the sample at the test moment or how well it is hacked up are variables. From a December Lancet journal article discussing false positives: “Technical problems including contamination during sampling (e.g. a swab accidentally touches a contaminated glove or surface), contamination by PCR amplicons, reagent contamination, sample cross-contamination, and cross reactions with other viruses or genetic material could be responsible for false positives.” Reagent contamination spoiled the CDC’s initial testing kits in March 2020.

The other point about PCR tests is discussed at length in an MIT/Mass. Institute of Technology medical services Nov. 20, 2020, publication. The highly complicated lab process is to “amplify or multiply any viral RNA snippets to a large enough quantity to measure. This test uses 40 cycles protocol. If the virus isn’t detected by then, the test is “negative”. And what is not reported yet is how many cycles it took to find the RNA. A positive test after 20 cycles contains more virus than one that took 40. But it can’t tell for certain if the virus is infective or not. And different machines can produce different cycle values from the same sample!?

For the public health medical aspect of the pandemic at this point, we have to accept the result as valid. The focus of the MIT article was a person who tested PCR positive, and said none of their family had symptoms, and they were feeling frustrated putting their lives on hold. But if someone dies, his or her life is put on hold and everyone else’s is changed permanently.

We must repeat the oh-so hackneyed saws of masking, distancing at least 6 feet apart (which could be a little too close for those of Scandinavian heritage) and hand washing for general purposes. Did you ever think you would feel positive about being negative and negative about being positive?


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