RT-PCR testing involves collecting a sample from the nose and throat using a swab. This sample is then sent to a laboratory for testing, where the RNA is isolated and applied to other ingredients that recognise and bind to SARS-CoV-2. The following reaction, or lack thereof, is used to diagnose the presence of the virus.
Once the sample is received at the laboratory, it is treated with chemical solutions to strip away proteins and fats, extracting only the RNA that is present. This RNA contains a mix of the person’s own genetic material and, if present, the virus’s RNA.
Using a particular enzyme, the RNA is then reverse transcribed to DNA using a particular enzyme. After this, scientists add short fragments of complementary DNA to specific sections of the transcribed viral DNA.
If the virus is present, the fragments attach to target sections of the viral DNA. Some of the added genetic fragments are used for building the DNA and adding marker labels to the strands. These marker labels are used to detect the virus.
The mixture is placed in an RT–PCR machine, intermittently heating and cooling in order to trigger chemical reactions that create identical copies. A standard RT–PCR sequence usually goes through 35 cycles. At the end of the process, around 35 billion new copies are created from each strand of the virus present in the sample.
As copies of the viral DNA sections are made, marker labels attach to the DNA strands, releasing a fluorescent dye. The machine’s computer measures the reaction, tracking the amount of fluorescence after each cycle. Presence of the virus is confirmed when a certain level is reached.
Scientists also measure the severity of the infection by looking at how many cycles it takes to reach this level. The fewer the cycles, the more severe the infection is.