PCR false positives aren’t causing the high Covid-19 case count, despite what you’ve seen on social media

Multiple misleading claims about the effectiveness of coronavirus tests have begun circulating on social media following the start of a mass testing scheme in Liverpool, alleging the tests have given false positive results that are now pushing up the daily Covid-19 case count.

Liverpool became the first area to be placed in the top tier of restrictions this autumn, and had a weekly case rate of 410.4 per 100,000 people as of 25 October. A pilot mass testing scheme to track the spread of the virus in the city was rolled out last week, but after initial results returned a low rate of positive results, social media has been awash with claims over the efficacy of the PRC tests used across the country – which experts have refuted.

PCR tests, which return results within 48 hours, are regarded as the “gold standard” by epidemiologists. They are being used alongside lateral flow tests (rapid tests) in the Government scheme. But on Monday Liverpool’s mayor announced that after 23,170 people had been tested in the first week of the pilot, only 0.7 per cent were positive for Covid-19. This is significantly down from figures recorded between 28 October and 3 November, when the city’s positive testing rate was 13.3 per cent.

Tweets and social media posts have been circulating online since Monday suggesting the numbers do not make sense, with the messages gaining thousands of re-shares. Some have claimed that Liverpool’s outbreak earlier in the year may have been a symptom of false positive PCR test results.

But experts insist PCR tests are very accurate, and say concerns that there are large numbers of false positives are unfounded, instead stating that issues can arise with how the tests are administered, and when people get the tests.

What are PCR tests and how do they work?

PCR tests work by detecting the genetic information of the virus, the RNA, which is only present if someone is infected and Covid-19 is present. Since they are designed to detect viral material present in the respiratory tract, their purpose is to find an active infection.

They can take a day or more to deliver results as the sample must be sent off to a laboratory to be processed.

PCR tests are used as part of the Government’s testing programme, and have been verified by Public Health England to have over 95 per cent sensitivity and specificity, meaning under laboratory conditions they should never return an incorrect result in more than 5 per cent of cases – whether a false positive or false negative.

Scientists use the samples to measure the cycle threshold (Ct) – a semi-quantitative value that can broadly categorise the concentration of the viral material in a sample. A lower Ct usually means there is a higher concentration of the virus, while a higher Ct usually means a low concentration of it, but this is less accurate to measure than the detection of the viral matter.

Why false positives can’t really happen

PCR tests do not produce false positives in the way people are suggesting online. The issue lies with when people get the test and how infectious they are during that time.

The problem comes from the very short window in which a person has the virus and can transmit it to other people, around day 5 to 10 of their infection.

Naturally people may miss this five day period and get a test just after the peak of their infection. They will still receive a positive PCR test result as they still have the RNA in their body. They might take a second test a few days or a week later and be surprised to find it comes back negative and blame the accuracy of the test – the issue is the time they took it, not the test itself.

Danny Altmann, professor of immunology at Imperial College London, told i that the data is already out there on the accuracy of the PCR test “which is very faithful”. He argued his concern was that false negatives are a more legitimate concern because nasal and throat swabs are not being properly carried out, because if they are done properly, it can feel uncomfortable.

“My assumption has always been that we’re underestimating positives significantly because it’s not very pleasant to get a good enough swab needed to get a good sample,” he said. “It’s very invasive.”

If the PCR test returns a positive result, it means the person most certainly has viral matter in their system, but there is slightly less accuracy in determining if they are infectious, he added.

So what is causing the differences?

Justin Lessler is an Associate Professor of Infectious Disease Epidemiology at John Hopkins Bloomberg School of Public Health. He believes the issue around case differences with different testing systems is ultimately an “apple to oranges comparison”.

“The mass testing is based on the entire population, while the PCR tests are only of people who are seeking out tests due to symptoms or being exposed to another case,” he told i.

Because of this, everyone tested by PCR is much more likely to be infected than the average person.

“I think a better way to think about this data is cases detected per 100,000. Mass testing is finding 700 infections per 100,000 in the population, but the standard PCR testing protocol is only detecting 300 cases per 100,000,” he said.

“That means that PCR testing protocol is missing 400 out of every 700 cases, well over half.”

As previously mentioned, he puts this down to most people who are infected not getting a timely PCR test, either because they don’t have significant symptoms or they wait too long to get tested.

“The reason the PCR rate is so high is that only about 2,200 per 100,000 people are getting tested by PCR, and those people are the people in the population most likely to be infected.”

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