There is an idea circulating that many of the positive PCR tests results for the SARS-CoV-2 virus in Switzerland are false, results which are known as false positives. However, according to experts interviewed by RTS, PCR test false positives are rare.
A number of political figures, including some Swiss parliamentarians, have been referring to large numbers of false positive SARS-CoV-2 PCR test results. While some infected with the virus are asymptomatic, this does not mean their positive test results are false, explain disease experts.
Confusion with other coronaviruses?
The PCR (pour Polymerase Chain Reaction) test involves taking a viral RNA sample from someone and testing it for specific genetic material. In Switzerland, the majority of laboratories use tests that target two, sometimes three different SARS-CoV-2 genes. Confusion could happen in theory but in practice in it doesn’t, according to Gilbert Greub, head of the Microbiology Institute at the CHUV hospital in Lausanne.
Since January 2020, SARS-CoV-2 PCR tests have been conducted on patients with coronavirus colds to test the test’s accuracy. Not a single patient with a coronavirus cold tested positive for SARS-CoV-2, according to Greub.
The PCR test is not perfect. False positives can occur when a sample is contaminated. However, this can be avoided with careful testing and lab work. Cross reactions with other genetic material cannot be ruled out either.
Test too sensitive?
Some criticise the tests for being too sensitive. While it is true that PCR testing amplifies SARS-CoV-2 viral genetic material to aid in the testing process, this does not mean there was no SARS-CoV-2 present in a patient testing positive. The process involves replication. Each replication is called a cycle. The more cycles required to detect viral presence, the less virus is present.
Commonly used machines perform 40 cycles of amplification. If more that PCR 37 cycles are required before the virus is detected, this is typically considered a negative result. Because of the specificity of the test, detection above 37 cycles suggests suspicion of infection, however, the detected material is likely to be dead and these patients are unlikely to be infectious. In some cases, this might be because the test was done too early or too late.
It is also true that not all positives are equal when it comes to viral load and infectiousness. Some might have a higher viral load or be at a more infectious phase of disease’s progression.
Distinguishing between those with high and low viral loads could be useful for identifying those most likely to spread the disease. Recording this information along with the results of positive tests makes good sense. This could then be used, for example, to focus limited contact tracing resources on those most likely to spread the disease.
It is also the case that a minimum number of viable viral particles are required to cause disease. One recent study suggests a few hundred particles of the SARS-CoV-2 virus might be enough. Another study suggests a sneeze from a highly infected individual could expel tens of millions of SARS-CoV-2 virus particles.
Laurence Senn, an infectious disease expert at the CHUV hospital in Lausanne, where positive PCR test viral loads are recorded, said that the vast majority of patients currently testing positive have significant viral loads. They would test positive even with less sensitive tests, she said.
More on this:
RTS article (in French) – Take a 5 minute French test now
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