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FactCheck: No, a claim by an Irish doctor that PCR tests have a 97% false positive rate is not true

The video, which has attracted hundreds of shares and likes on Facebook, makes a number of claims about PCR testing.

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A VIDEO WHICH claims that the PCR test used to detect the pandemic coronavirus is inaccurate has been viewed over 6,000 times and attracted hundreds of shares and likes on Facebook. 

In his video, Dr Vincent Carroll described the test as unreliable and said it should not be used.

“It has a false-positive rate of 97%,” he said. “It is a completely misleading test.” False positives are when test results show someone as having the virus when they do not.

Dr Carroll, a GP in Sandymount, Dublin, referenced a legal case in Portugal to support his views. 

The legal case questioned the results from PCR tests, though the judgement itself proved controversial among medical experts.   

“You get a lot of false positives because you start to identify things that are nothing to do with the current infection,” Dr Carroll said. “It might be fragments of DNA from other coronaviruses.” This happens because the original sample is amplified “through too many cycles”, he added. 

Dr Carroll, who has a master’s degree in public health medicine, says that a positive PCR test does not correspond to an active case of Covid-19.

The test

The PCR test is considered the gold standard for diagnosing a suspected case of Covid-19, and is the test recommended by the European Centre for Disease Prevention and Control and the World Health Organisation.

PCR (polymerase chain reaction) is a way of multiplying pieces of genetic material from a germ, which is tiny, in order to study it and detect the presence of the genetic material of a virus. It can either say that a virus is present or that it is not present. 

During each cycle, any genetic material from SARS-CoV-2 is doubled, until there is enough of it to be detected in the test or the test ends with a negative result.

PCR only works with DNA, so first viral RNA in the nasal swab is converted into DNA. Then the DNA is unzipped to give two single strands. Scientists add special codes of DNA (primers) that match the germ of interest. 

Only when there is a match can a special enzyme start pairing up DNA, doubling that DNA during each cycle. If enough DNA is made, a signal is detected by the PCR machine and the PCR test is positive. If nothing is detected at this stage, the test result is that no virus is present. Most commercial test kits stop after 40 cycles. 

Experts in PCR contacted by  TheJournal.ie  viewed the 97% figure for false positives as a misunderstanding.

“This is not true. This demonstrates a clear misunderstanding of the PCR primers,” says Michael Mina, a professor at the Harvard T.H. Chan School of Public Health. 

Primers are the reason why the PCR test for SARS-CoV-2 will only pick up that specific virus and not any other coronaviruses. 

“When setting up a PCR test, you need to know what you are looking for, otherwise you won’t amplify anything,” says Dr Cillian De Gascun, director of the National Virus Reference Laboratory in UCD. 

This was a disadvantage when the virus first emerged in China, as doctors there began seeing patients with a peculiar pneumonia in their hospitals. “They used PCR to test for influenza and other [known] respiratory viruses, but the problem is PCR is really specific,” says Dr De Gascun. The standard test cannot recognise unknown germs. 

Only when the entire sequence of the new virus was fished out using special bioinformatics software could a PCR test for SARS-CoV-2 be set up. Scientists then rushed to make probes that would match only this virus – and not other germs. Dr De Gascun says the true false positive rate is between 0.1 and 0.2%.  

Contacted by TheJournal.ie, Dr Carroll pointed to two scientific publications, by a French group and by a Canadian group. “Anything above 25 cycles [of PCR], you get enormous numbers of false positives,” he adds, referencing the Canadian report.  

Dr Carroll argues that “the only thing that really matters when talking about population health is whether a person can pass the infection on from one person to another”. 

He describes false positives as “when a PCR does not correspond to an active Covid case”, but PCR experts disagree.  The test is a yes or no for whether SARS-CoV-2 is present in a sample. A false positive is when the test wrongly says the virus is there. 

In Canada, Dr Jared Bullard who wrote the Canadian paper says he has been intrigued by the attention his research got on social media, first by scientists, then by the lay public.  He is an expert in PCR testing for viruses at the Children’s Hospital Research Institute in Manitoba and associate medical director of the main viral testing lab for the province of Manitoba. 

His research paper showed that after day 8 of symptoms, it is difficult to grow SARS-CoV-2 on cells in a lab. “The take-home message was that you are probably no longer infectious and so we can remove you from isolation,” says Dr Bullard. Isolation policies around the world shifted partly because of his findings. 

Dr Bullard is surprised to see discussions of cycle values (CTs) and his paper, but adds that “it is not accurate to say that we have false positives if we have a CT greater than 25.” He has seen this misinterpretation of his paper on social media in Canada and the US.   

He says if the test requires more than 25 cycles, then there is not a lot of virus present.

This means that the person is either at very early stage of infection, possibly day one or two, or they have mostly cleared the infection. 

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“You can have people closer to day 10 or 14 of infection,” says Dr Bullard. “But they definitely have Covid and it is important to know, because we can and should follow up on cases and contacts of that person.”

Even if they were infectious in the past, it allows contract tracers to “hone in on when we think people will have been in contact with that individual”. Likewise, “if someone is saying my symptoms just developed yesterday, we know we may have found them before they became very infectious.” 

“Of course PCR positive tests are not false positive in 97% of cases,” says Dr Bernard La Scola, the infectious disease scientist at IHU Mediterranee Infection-Aix Marseille University in France, who co-wrote the paper from the French group that Dr Carroll cites.

In our paper, “we show that below 35 cycles it is possible to cultivate the virus in secretions and thus that the patient is potentially contagious,” Dr La Scola told TheJournal.ie.    

After 35 cycles, the person may be positive on the PCR test but now cured. During a massive epidemic, Dr Scola continues, “it is a waste of time to consider these rare cases.”  

Dr Carroll claims the PCR tests has led to unnecessary lockdowns. He advocates antigen testing as an alternative, describing it as rapid and inexpensive, in an interview with TheJournal.ie.

“Antigen tests have too much false negative,” counters Dr La Scola, meaning you wrongly give people infected with the virus the all clear. “PCR is the best way to count cases,” adds the French expert.

Dr Bullard says the place of antigen testing is still under discussion by experts, and the test might be useful in care homes to routinely test workers, for example. It is not so reliable in showing that a person does not have the pandemic virus.

A person with a positive PCR test may not necessarily be sick or infectious. Instead, the test is a snapshot in time that says yes or no to the presence of SARS-CoV-2

Professor Mina calls those who are detected after they have been infectious ‘late positives’. 

“The PCR can detect virus in people after they have stopped transmitting the virus,” he notes. These people may have passed it on to others in the past though.

A positive PCR result confirms that the person has had Covid-19, even if they were asymptomatic, according to Dr Bullard in Canada, and is a useful finding.

Verdict

The video is wrong that the PCR test for SARS-CoV-2 gives a false positive rate of 97%. Also, PCR tests do not wrongly confuse genetic material from other viruses with SARS-CoV-2. 

The scientific consensus is that PCR tests are the best option for public health screening right now.   

Dr Cillian de Gascun of the National Virus Reference Laboratory in UCD says the actual false positive rate is between 0.1% and 0.2%.

As a result, we rate the claim that PCR tests have a 97% false positive rate as FALSE. 

As per our verdict guide, this means: The claim is inaccurate. 

TheJournal.ie’s FactCheck is a signatory to the International Fact-Checking Network’s Code of Principles. You can read it here. For information on how FactCheck works, what the verdicts mean, and how you can take part, check out our Reader’s Guide here. You can read about the team of editors and reporters who work on the factchecks here.

About the author:

Anthony King  / Freelance science journalist in Dublin

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