How a PCR test works

A PCR test is a widely-used, highly sensitive test that seeks out traces of genetic material from a specific pathogen, if the pathogen is present in the body. The technique is a powerful diagnostic tool that can identify both DNA or RNA from specific microorganisms, irrespective of whether they are bacteria or viruses.

A PCR can capture a specific gene from genetic material (DNA or RNA) in a swab sample, and multiply it through a series of chemical processes so it can be detected using fluorescent dyes.

RT-PCR, a variation of the PCR test

The version of PCR testing used to detect viruses like the COVID-19-causing SARS-CoV-2 is called RT-PCR (reverse-transcription PCR).

While some viruses have only DNA, others like SARS-CoV-2, only contain RNA. Viruses infect a healthy cell and hack into the natural machinery that cells use to process our own RNA, so the virus particle can multiply and survive. Once inside a cell, the virus uses its RNA to take control of the cell's machinery and ‘reprogramme’ the cell into a virus-making factory.

To detect an RNA virus, like SARS-CoV-2, scientists use an enzyme (reverse transcriptase) to convert the virus's RNA into DNA, in a simple and widely-used one-step process called ‘reverse transcription’. This allows a single molecule of DNA to be amplified exponentially (millions of times), which is the main goal of the PCR process, so even virus particles in single digits can be detected in the final result.

An RT-PCR test is considered very reliable because it can detect even a single virus particle in swabs taken from inside the mouth or nose, where the virus particles are most prevalent.

Advantages of RT-PCR testing

Swift, sensitive and specific:

There are real-time variations of the RT–PCR technique that are quick, super sensitive, and specific. They can deliver a reliable diagnosis in three hours. That said, the process on average takes around eight hours to produce a conclusive result.

More accurate, less prone to contamination:

Compared to other methods to isolate and detect viruses, RT–PCR is much faster and less likely to be contaminated or cause errors during the testing process. This is because the test – start to finish – is carried out in an enclosed tube, inside a specialised, automated machine.

Real-time RT-PCR continues to be the most accurate method to detect the SARS-CoV-2 virus.

Severity of infection can be estimated:

A standard real-time RT–PCR set-up usually goes through 35 cycles, which means that, by the end of the process, around 35 billion new copies of the sections of viral DNA are created from each strand of the virus present in the sample.

Covid Test