How does a RT-PCR test for COVID-19 work?

A video recently shared on Facebook made the claim (at 05:39): “[The] problem with PCR [testing] is that it’s something of a haystack creator in the search for the needle that should be very obvious.”

This echoes a previous claim that we fact checked, which claimed that a high number of amplification cycles used in an RT-PCR test will lead to a positive diagnosis of COVID-19. This is simply not how PCR testing works. 

So, how does RT-PCR testing work?

The RT-PCR test detects the presence of SARS-CoV-2 (the virus which causes COVID-19). The test looks for evidence that the virus is currently in your body, by detecting the presence of its RNA in a swab sample from your nose/throat. 

The reverse transcription polymerase chain reaction (RT-PCR) laboratory technique is used to test for a positive diagnosis of COVID-19. Real-time RT-PCR is the preferred method of testing for a positive diagnosis of COVID-19 in the NHS, as is the case across the world. It is considered to be one of the fastest and most accurate laboratory methods for detecting the SARS-CoV-2 coronavirus (the virus that causes COVID-19).

In effect, the PCR test detects the genetic material in the virus called RNA. In the lab, a solution known as a ‘reagent’ is added to the sample. If the virus is present, this reagent starts a ‘chain reaction’ and creates billions of copies of the genetic material in the virus so that there is enough for scientists to analyse and determine a result.

Typically,  the process goes through 40 cycles, which will create billions of copies of any viral DNA strand in the sample. As new copies of viral DNA are made, these will release a fluorescent dye, which can be monitored in real time by the person running the test.

When a certain level of fluorescence is met, this tells the scientist running the test that viral RNA was present in the original sample. The number of cycles required before the virus can be identified is known as the cycle threshold (Ct).

How accurate are PCR tests?

Claiming that anyone can generate a positive test by running a sufficient number of cycles of amplification — instead of 40 as usual — to create a “haystack” in which to find a COVID needle is not an accurate representation of the processes or the parameters which labs work within.

Protocols state that 40 is the maximum number of cycles, with some countries also stipulating that a Ct of 38, 39 or 40 should be regarded as an ambiguous result. For tests carried out by the NHS in the UK, an individual will be notified of a positive test result only where the Ct is 40 or lower.  

PCR tests have been described as “extremely accurate”. That’s not to say that they are not without their issues.  There have been incidences of false negatives (giving a negative result for a person infected with COVID-19) and false positives (giving a positive result for a person not infected with COVID-19). However, research on these issues has found that under laboratory conditions PCR tests for COVID-19 “should never show more than 5% false positives or 5% false negatives”.


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